[sanddrag]

Well, you can get a 1/2" ID 1.125 OD flanged bearing either from Small Parts (p/n BRF-08) or from AndyMark. I'm not too sold on the idea of putting that wheel on just one bearing though. Here's that might work. Make some standoffs, (round bar stock with hole in the middle) maybe about 1" long and put like 6 of them on the wheel (every other hole). You might be able to get by with nylon ones from the hardware store. Then, buy a sprocket big enough that you can put the same hole pattern in it. Also, bore out the center to 1.125" If you do not have access to machine tools, the hole pattern can be done with a drill press and a bit of careful planning and measuring but the 1.125 bore might need to be machined. Bolt the sprocket to the wheel with the standoffs in between. Put a bearing in the wheel. Put a bearing in the sprocket. Call it done.

Another thing you could do, if the bolt circle on the wheel is too big for the sprocket of your liking, would be to run a live axle. Make some 1/8" steel or titanium plates with something like a 1/2" hole and an 1/8" keyway. You can lasercut the whole thing, or you can cut the plate some other way and brach the keyway (or dremel if you are desperate). Bolt one plate to each side of the wheel. Use a 1/2" keyed axle and a 1/2" keyed sprocket.

If you have access to a machine shop, you can make some really nice aluminum hubs for the wheel.

So buy sprockets, www.mcmaster.com and www.sdp-si.com are good places to start your search.

EDIT: AndyMark sells a wheel hub 1/2" ID with 1/8" key for $16