MSC, Enco, McMaster-Carr - Enco, particularly, has lots of sales/free shipping deals (and is apparently owned by MSC for some time now.) Import .vs. Made in USA is clearly called out (and once in a while is actually India, rather than that other place) so no problem/mystery choosing domestic production, even though they certainly have lots of import stuff. Enco has a bit less selection than the other two.
There are many others, those happen to be the big three, and get stuff to you in reasonable time, if not always the rock-bottomest of price for everything - shop around before committing - but often they are the only places you can find that have certain items (now, things like a Sawzall, you'll laugh at MSC's prices for, but those are easy to get elsewhere.)
I'll certainly go have a look at Travers now that Swingman has mentioned it - was not on my radar previously. Comparing an item I use, Enco hot deals catalog for the month has Atrax USA long 1/2 solid carbide end mills for about $8 less than Traver's TTC offering (and $70 less than the MA Ford version) for the 2" flute length, and has a 3" flute length version if you need it (be careful about CNC Router programming errors or you'll snap that sucker) which I don't see at Travers. Can't say that I've actually compared all three brands in use. Regular prices are about the same, but the sales are recurrent at Enco.