I purchased mine from J-P Cycles.  $98 for the short ones and $102 for the long ones.  There are some also some cheaper Custom Chrome risers available (about $55) which are comparable to the 971's, but with a little less rise.

LaMonster risers:

LaMonster sells some very nice looking risers, which provide an additional 1.25" pullback and .25" rise.  These are available from Hal at the VRCC Store.  Note: The older version of the LaMonster risers (shown in the above photos courtesy of Dennis Finn) did not cover the indent around the holes in the triple trees.  The new version has apparently addressed this issue, completely covering the holes.  However, the stock top clamp can apparently no longer be used.

H-D Deuce risers:

According to Mase: "These are from a HD Deuce. Part #56033-00 $149. They rise 5.5" and pullback 2.5". You will have to reroute the throttle cables to the inside of the fork tree. Also, you will need to cut the tab off the bottom (easy to do) and get different length bolts but that is it. It should only take about an hour and a few beers (finish the rest when yer done!)."

J-P Cycles now sells a Deuce riser clone for $75.

Cycle House risers:

This is one of the longer-than-stock Cycle House risers (they are advertised as "4 1/4 inch rise & 1 inch pullback"). 

installation:

The Custom Chrome risers (or any Harley-based risers, for that matter) require that the triple clamp be drilled out to 1/2".  The existing holes are 12mm, so only a tiny amount of material needs to be removed.  You can always go back to stock later.  You should apply several layers of duct tape or masking tape around the holes in case you slip when drilling.

Also for any Harley-based risers, you must also purchase some 1/2" 20 by 1.5" long Grade 8 (hardened) bolts.  You might as well have some washers on hand in case the risers are not threaded deep enough.  Either use Loctite or lock washers (or both).

Tip: put a pillow on your tank for a riser swap.  It also helps to have a second set of hands.  I cursed like a sailor several times.

clearance issues:

The throttle cables may have to be rerouted to behind the triple clamp.  I cannot say for sure, as I have aftermarket throttle cables that are longer than stock and Two Brothers Racing triple trees, which may vary slightly from stock riser mounting holes.

I have not had an opportunity to try out the longer risers (08-972), but they will probably require extensive cable rerouting.  LaMonster reported that he never could get his throttle to work right with the longer risers, but others have reported success.

The smaller risers (971) just barely fit with an Interstate tank (at least with the Two Bros clamps), allowing the control cables on the left to barely touch the tank at full left lock.  Just eyeballing, the longer risers (972) should clear just fine.