I got them from Princess Auto. I'm not sure if you have any stores in the US, but you can buy them online. They were $15 for four and are very well made. Power fist brand.
http://www.princessauto.com/pal/en/Clamps/4-pc-Butt-Welding-Clamp-Set/8393878.p
These are also very handy.
http://www.princessauto.com/pal/en/Clamps/4-pc-Micro-Welding-Clamp-Set/8393845.p