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.