almost all the hubs these days take the same size 3 tabbed sprocket. (bendix is slightly smaller, sometimes they fit other hubs, sometimes they don't.
search ebay for coaster brake cog (or sprocket) and look for the ones with 3 tabs.
an older bike shop should have piles of them laying around, too.
one thing about a larger rear cog, your coaster brake will work a lot better. i switched from an 18 to a 20 and almost launched off my bike the first stop. it'll lock up a lot easier.
or you can go even smaller on the front sprocket. here's a 32 Wald:
Wald 32t Sprocket Chainring New - eBay (item 360302090960 end time Jan-16-11 18:05:04 PST)