Yeah it would probably be best to wait until you can afford a larger tank. They aren't expensive, its about 10 bucks for a nice very large 10 gallon. How large is the heater you have now? Depending on how many watts the heater is and your climate, I would worry about it over heating a half gallon tank. I know where I live with my tanks (which are on the smaller size, 3 and 1) I don't need a heater in the summer.
If you must get him now, you can get inexpensive tanks (like critter keepers) in larger sizes. You can also check your local thrift store, I know my Salvation Army usually has some tanks that are very cheap. Just be sure to wash it very, very well. Another option is looking at your dollar store for a LARGE vase. Not a tiny one, but a large one that will give him room to move around. Or a storage container, try stores like Big Lots, Walmart or dollar stores for cheap ones that are around 9-10L (which is about 2 1/2 gallons)
I wouldn't keep him in a 1/2 gallon, he would be much happier in a larger tank. (at least 2 1/2 gallons)
Don't forget to post pictures when you get him :]