I have a 30 year old Bianchi Foderi Forcella, and i kinda banged the pedal really hard on the ground, causing the pedal to break. so when i went to replace it, i found that the threads on the crank arm are stripped. would any one know where i can buy just the crank arm, preferably for under 50 dollars. thnks

You could have new threads via a helicoil, too...

ebay. Whenever I have searched for cranks, I have often found listings for single arms. Also, if the damaged crank is from the non-drive side, your cranks are 170mm long and you don't mind mismatched crank arms, I have a single nds sugino arm lying around shipped for the price of postage.

No its on the geared side, i think i have decided to just buy a new crank set. but i am just wondering if i can buy just any 170mm crankset or are there specific sizes that have to be right just to get it to fit?

Depends if you want to use your current bottom bracket, do you know what crank it is?