Stewmac, Dremel, MLCS, or Whiteside, will have them. You might also keyword search "1/8 diameter end mills". A lot of places that sell, and service bits for CNC will have them. Plus just searching will get you lower dollar options like these. Amazon has some too. I will spend a few bux on one that has a cut configuration of what I want sometimes. Not looking at them for hours of work, there I will go higher dollar stuff, but for small runs, or one off...

I am just setting up a cordless router in a table to do some 1/8" diameter work, there are plenty of places out there. seems more growth for 1/8" stuff than any other shank size right now.