Eric.. Norcal Airgas...Sierra AIRGAS should have them where you live...
I use Norton brand,, they are spendy but have several grades,,, the good ones hold up really well....
you can get them at harbor frieght,,,I dont recommend theirs they are junk...
they make magnetic squaring clamps.. they are faily cheap (harbor freight) or better ones at the welding supply,,, I like the american made ones,,, the magnets are stronger
I use BESSEY clamps for my work with an ACORN table for big stuff...
but its really just some experience with the welding gun... neither of the above is needed... if things are truely square when tacked (properly) they shouldnt move enough to get out of square...
Ill bet youll find you arent getting things as square as you think... or they are moving before/while you are tacking them and you arent rechecking...
This is a basic fab technique.... practice makes perfection
Good luck
cam