I know you can buy a million o-rings at harbor freight for a few dollars; but if you want to avoid sorting thru all the odd sizes, Home Depot and Lowes have the correct size O-Rings to fill in the gap for the air-ride tires (on NSR and AllSlot wheels). Go to the plumbing aisle to the washer section. Look for #41 and #7 O-rings. Ten-packs are $2.30.
#41 has 7/16in inside dia, for the rear wheels. #7 has a 3/8in ID for the AllSlot fronts (NSR fronts have a solid nub on the hub.) Both are about 1/16 thick – it takes about 3 to fill in the groove for each wheel.