Charlie: Look in your yellow pages phone directory under "Wheels". You will find them listed after "wheelchairs". The word "tire" implies that it's a rubber pnuematic wheel for a vehicle of some description, whereas the word "wheel" doesn't have that implication.

Phone the places listed under wheels because many of them will just specialize in chrome plated wheels for hot cars, or hubcaps. But, some of them will specialize in wheels and casters, and that includes small pneumatic tires for trailers, golf carts, and hand trucks (or as you say "dolly").

You should be able to buy a bolt together hub and a inner tube to fit your existing wheels, but the difficulty of getting your existing rubber off your existing hubs is another matter. You may end up buying new tires as well, and by that time, you're spending several times what the "dolly" cost you. You'd have been better off buying a well made hand truck right from the start, one that's made to be servicable.

Another option would be to forget about the pneumatic tires and simply buy solid wheels for the hand truck of the same axle diameter. Most well built solid wheels accept axle bearings of different sizes so that they can be made to fit different size axles, like 1/2, 5/8 or 3/4 inch diameter. With solid wheels, you never have to worry that the OTHER wheel will go flat.

I'd throw the hand truck in the back seat of your car and take it down to your local wheel shop and see what they can put together for you at a reasonable price. (I'm guessing you only paid $19.99 for the entire hand truck to start with, and so you don't want to spend anywhere near half that in fixing it, and therein lies the peril in buying cheap stuff that's not made to be servicable.)