If you had trouble getting the tires off, chances are that you need a new axle, also.
If possible weigh your rig before buying tires.
If you want something that looks stock, I'd suggest this wheel and the accompanying hubcap:
15" Trailer Wheel
Reports on fitment vary from one trailer to the next. You may be able to fit 225/75R15 trailer tires, which would give you the most capacity. If they would rub you may have to use 215/75R15 or 195/70R15 light truck tires.
There are limited selections of brands in these sizes. I have Goodyear Marathons, which have worked out for me, but lots of people hate them.