Although not a American food store, B&M has a lot of random American food. Last time I went I spied: Twinkies, malt o meal version of lucky charms (and some other cereals), giant boxes of Mike and Ike and movie snack boxes of butterfingers, cookie dough, reese's pieces). Everything was a lot cheaper than the price you'll find in a specialty shop.
I wouldn't make a special trip to B&M, but if you are near one it might be worth a look.
Poundland also consistently seems to have nerds, runts, butterfingers, reese's pieces, cookie dough and other movie box size sweets now.