Michelle, (I had to go back and read the thread) The wheels you are describing 8" OD, 4" hub, are totally standard dolly tires. Compare them to tires marked 4.00/3.50-4 What you need to do is put a tube in them. I sold these things at The Hardware Store for years. People found it especially tough when they had paid $32.99 for a dolly and were told it would be $26 for a new wheel with tire or $7.99, but all the tubeless tires they put on these things start losing air after a while - corrosion and stuff - and then are really bad at losing it fast. Putting a tube in, as I did for example on my wheelbarrow stored outside, changed the loss of air from a couple days to months. Pneumatic tires are nice because they absorb little bumps and squish over obsticles - they will not get hung up by falling into a crack in the pavement. You have discovered the downside. Casters are anything that steers by castering - the center of force is off set from the center of swivel and when you push they steer/pivot until straight. They are hell with a heavy load on a tilted surface, like a sidewalk or driveway that slopes to one side, because they want to go straight down hill. That is the reason that heavy material handling platforms have two fixed and two or more caster wheels. (At Home Depot the wood handling carts have two fixed in the middle and casters at each end.) Casters can be bought in all kinds of ratings. We sold a 4" wide, 4" diameter, 6" tall (2" for the mounting plate and bearings) solid plastic caster rated at 900# per caster. It cost about $35 each. The smaller the diameter, the more it will hang up on cracks. The narrower the more it will sink in to soft stuff. The bigger, the heavier it will be - pneumatic less so. If you look around, you can find solid foam (filled or molded) wheels that give the same flex as pneumatic and about the same weight. Unfortunately, they end up costing twice what pneumatic tires cost either total cost for the molded or the fact that most places that do the foam filling will not do it with used wheels and tires - they start with a new set and charge about the same for the foaming. Go to a serious caster company that is helpful, some only want part numbers, and take a look at choices. You will probably be able to match your casters to tubes there. This is something they do every day. When I get expensive casters, I mount them separately to boards for each end or for a full plate, and bolt them to the box from the outside, so I can remove the plate for moving other stuff, get the weight off the casters when the box is not going anyplace and get the casters up out of the damp and dust. A set of casters costing $100 total should be treated like other tools that cost $100.