You could buy some store bought toys. Some good ones are hanging bird toys to bat at and chew on and ferret tubes and Roll-A-Nests. It's also good to visit hardware stores. A lot of expensive toys can be substituted with cheaper equivalents like PVC piping, ropes (no sisal because it makes rats sneeze), and plastic flowerpots (which can be made into igloos).
It's also pretty easy to make your own toys. For instance, if you want to make tubes or balls, you could cut up some large plastic pop bottles and juice jugs. You can also make hammocks for your rats with polar fleece. If you don't have any sitting around at home, you can probably get some cheap stuff from any fabric store.
Also, you should see about getting at least two rats. Rats like to be together, and since you won't be able to play with him 24/7, you should definitely look into getting him a playmate.
Hope that helped!