I frankly forget where I got them and how much but here is a set on ebay for 4 for $165 shipped exactly like mine:
http://cgi.ebay.com/ebaymotors/4-12-TIRE-SKATES-WHEEL-CAR-DOLLY-BALL-BEARINGS-SKATE_W0QQcmdZViewItemQQhashZitem4a9c71b623QQitemZ 320452277795QQptZMotorsQ5fAutomotiveQ5fTools
I considered the hydraulic type but figured that might be 4 more things (pumps) that I would have to potentially fail. It takes me all of 5 or so minutes to jack all four corners up and place the skates under each wheel. I hang them on pegs when not in use. These are VERY heavy gauge steel and much more substantial than what I have seen at HarborFreight.