I used some cupped washers. You can find them at Lowe's or Home Depot, or a lot of hardware stores.
Also, you wheel (rim) should be pretty straight or you're going to have other problems real quick.
If the sprocket is not centered perfectly on the hub, that's not a huge problem....make sure you have 3/4"-1" of slack AT THE TIGHTEST POINT.
If the sprocket is moving side to side, just retighten all the biolts in a cross pattern going just a turn or so at a time and get that bugger straight.