Our free, plentiful favorite for storing wheels is the long arm of a side pull brake.

It has a hole for a big headed nail or screw. You can use a washer to keep nail/screw from pulling through.

This hook is strong and can hold one wheel, or two thinner rimmed wheels.

With the arm twist, you have to pay attention to the direction orientation, and sometimes we modify the twist, using a vice.

With the slight lip at the top of the arm, it is best to afix this type of hook on the edge of a wood beam, although the arms/hooks work just fine on a blank, flat wall too.

A few pix down on linked page you can see some in action. They are on one of our wheel tripods, which we have modified since the picture.

http://strans.org/communityshop.html

We also have a few hundred wheels on a tall wall in our ped tech warehouse- in that case we used 20d nails I think, and nailed at an angle into the studs. For that application, we used only axleless wheels that would lay flatter against the wall.