I'm assuming you have the HP TouchSmart (IQ770?). I've heard good things about the touch screen part, but not so good things about the computer itself.
http://trolltouch.com/ and
http://www.magictouch.com make some for the Macs, but they're a little expensive.
http://www.superwarehouse.com and
http://www.tigerdirect.com sell just the monitors, as do others, but it's not as good of a solution. Planar makes some nice ones though.
I don't know if that helps any for when you're ready to buy (new Macs in June probably) but like I said, I have a handicapped friend who wanted me to help her look into this, so I've been doing some research.
Get a refurb model to save some money. The new MB will have a better video card and faster FSB, same for the 24". If the current models are ok to you, buy it now. Otherwise, be prepared to wait until June.