Sure, you can buy an aftermarket shifter. You can probably ask your local bike shop to order you a cheap shifter. They can check out the ratio for you, decide whether to order something like a Shimano Altus vs. SRAM X.3 (I think those are the two companies' least expensive offerings?), and then install it for you. But here's the catch: 1) that part + installation will probably cost you more than the bike is worth, and 2) you'll be out for another hour and something else will break. I know you said you don't have a lot of money, but even an old fully rigid bike off Craigslist will likely be an improvement over what you have. If you tell us where you are, we can check Craigslist for you.