Have a look at Jurassic Tools. They have a huge range of 1/8" shank cutters.
They are a bit hard to find on the site. Select Milling Cutters from the menu and put them in price order, low to high. Most of the first stuff on the list is 1/8" shank. They are about £3-£4 each.
I've got a few of these and they are great on mild steel and brass. I've used them on silver steel as well with light cuts.
I've not tried the carbide ones from there though.
Steve