8" is a bit small compared to what the hardware stores around here tend to stock. You'll have an easier time finding 10"+, but that limits your wheel configuration to 4wd, or a really narrow 6wd, and both of those come with their own set of problems. Apart from home depot, princess auto also carries a selection of tires. if the right size is not available locally you can order from Mcmaster-Carr, they are in the US but shipping is really reasonable and fast. They carry a huge selection of tires, it's not cheaper, but it's always going to be in stock.
http://www.mcmaster.com/#pneumatic-tires/=10ojbe7
Although the Andymark wheels are expensive to get to us, and are not in stock atm, the fact that the hubs use the FRC standard bolt pattern means they can easily be attached to the belts and sprockets that vex and andymark have available. Converting other pneumatic wheels requires you to do one of following:
Make the standard 6 hole bolt pattern on the hub (which may already have a 4 bolt pattern on it)
or
modifying your sprockets/chains/gears to fit the existing pattern on the hub
Both options work, but if you do choose to do this know that it does take some time to fabricate. Care needs to be taken to keep them as concentric as possible unless you don't mind a lumpy robot, definitely something that should be done on a mill if possible.