I didn't buy anything from KMS / BB in respect to end mills since I started this hobby like 5 years ago or so. I think I bought a grand total of... like 4.
So given that I am now an owner of ... I have no clue like a 1000... I disclose that I get them mostly on eBay or auctions locally. The remaining - say around 50 or so I got directly from China in metric sizes brand new long time ago (i.e. after the 4 I got at KMS/BB). Usually paid around $2 per end mill or at most $3 in sizes up to 12mm. For carbide 6mm-8mm brand new under $10 including shipping.
Through you can get a LOT of used HSS endmills for next to nothing off eBay do know that they will be dull so you may need to re-sharpen them or use them on aluminium or use them for "practice" etc. On the other hand you can also get brand new, US made end mills HSS / M42 on eBay as well which are a good deal.
You can get used carbide endmills on eBay. Say 3/4 used carbide endmill will run you like $5 plus shipping (buy a lot at once). They may be a bit dull but what happens is that CNC shop uses them and once they are run through pre-set time they are considered "dull" and sold off by a specialised seller that deals in surplus. They may not be that bad. The reason CNC shop is getting rid of them is that re-sharpening flutes may cost more then it is worth in changing their G-code - size shrinks a bit.
Expect to pay around $30 USD plus shipping for brand new carbide 3/4 end mill. quite more for a 1 inch carbide with fancy coating. This is still many, many times cheaper than retail.
Please note that professional shops mostly use carbide end-mills and carbide drills when running longer part runs. It is simply cheaper for them than changing tooling more often. However, they will still say use M42 drill if they have a small run of something. These shops get their stuff from large tool suppliers. KMS/BB has rather thin selection as they are emergency hobby only buy location. Unless you want to spend 3/4 times more it is just easier to skip the middle man and buy directly from China where BB/KMS gets their stuff - or get nice US made HSS/M42/Carbide off eBay brand new for less or same price as KMS HSS China stuff.
90% of all my stuff comes from KBC and overall I'm very happy. I mostly order their house brand which is a mix of Eastern Europe and China. Always feel that I've gotten my moneys worth. I pretty much stick to plain old HSS. I just don't work fast enough or with any kind of material that warrants the higher priced coated or carbide.
I've seen alot of people like Tom go China direct but I've never pulled the trigger myself. I prefer to let KBC do the quality control work for me and pay the couple of bucks extra.
Auctions, if you get lucky, can be a great source. Frequently you're paying near scrap for quality used stuff.
My project has required metric end mills. Initially I bought from KBC which is kind of my go-to supplier. Sometimes they are comparable, sometimes not. Like most things it pays to check around & of course depends on how much time you have to wait fro delivery. I have gotten some real nice stuff off Aliexpress & Ebay for substantially less. Especially in the smaller sizes like 1-8mm and particularly specialty cutters like ball end or roughers or aluminum cutters. I have had a few disappointments, but not many & not a lot of expense involved. I do think there is a grey secondary market out there that possibly buys rejects or non-spec. So sometimes they are ok & sometimes not. Try & stick with what looks like a dedicated/specialized tool supplier vs one that carries end mills, USB cords & Q-tips... Having said that, you can get burned locally just as easily. I have a box of Cdn Tire drill bits that are sold as surplus but are in fact factory bastards. Unequal lips, unsymmetrical flutes., incorrect relief. Absolute rip off crap. One day when I have photo magnification I'll post.
Metric end mills for the most part are shank = cutter diameter. So typically you would have something like an ER holder & graduated collets to accommodate. Whereas N-Am are more standardized IMP shanks which is both good & bad. So you might have 3/8" socket end mill holder to accommodate say 3/16=3/8 cutters which is good. To span a typical hobbyist range you likely will have a set of EM holders which can consume some $ too. Some guys use R8 collets directly in the quill, that has pros & cons. The IMP system kind of breaks down when you require metric cutters with IMP shanks. They are not as common & more expensive.
One technique I use a lot now is a matched pair of rougher & finishing EM's. Whether it's both 3/8 or both 1/2 doesn't really matter. But the general idea is use the rougher as its intended - higher removal rate, nicer chips, easier on the machine (especially smaller HP machines). Leave 10-thou for the finisher to achieve final dimension & finish while the part is set up for the operation & you have dimensions fresh in your brain. This also extends the life of finishing EM. Note the tolerance specs on the rougher may be different (worse) that typical EM, at least that's what I found, so substitute for checking dimensions along the way.
Yup I've shopped there. Jake is the owner, good guy. He has access to some nice brand names you might find more difficult to source elsewhere. And some things are the same as elsewhere. A few years ago I picked up my GS brand ER collet holder, some nice metric taps through him, they are awesome. If you are familiar with the Sowa catalog, I think that's one of the product lines he carries. I think mostly he serves machine shops but for sure welcomed me as a hobbyist. I cant say that prices are much different than competitors, it seems to be a dealership / margins game wherever you go.