Some decent cranksets on amazon that run from cheap on up...
http://www.amazon.com/crankarm-alloy...words=crankset
is one example. A simple sort for Crankset provides pages of replacement left side arms starting at $9.99 so should be simple to click on a few of them to find one that looks the most right. If you don't find the exact arm, then look read further for a full crankset... many starting at $18. Be sure to get a bottom bracket and the required tools also so you can get back on the saddle promptly...
Noting also that sorting the crankset search by Price "lowest to highest" not only finds arms but also lot of ads for the crank bolts and caps and tools...all of which can quickly let you figure out what yours actually is since most adds have hi-res photos and text descriptions that you can match up to your bike to make sure you make a knowledgable buy.