These are like the ones used here at Quest Air (on the tow line, of course):

These are $4.50 at the ACE hardware in Clermont. They are not marked at 500 kg here at Quest Air, nor at ACE. The site linked to if you click the picture above sells them at much less if you purchase large quantities.

They are very light, rugged, and if they break, easily replaced at little cost. They have experienced no problems with these carabineers on both ends of the tow lines here at Quest Air.

If you need a self locking lightweight carabineer you might try one of these:

http://www.camp.it/template01.aspx?codicemenu=75

Not nearly as light as the one above at 3.2857 ounces or 92 grams, much more expensive at $16, but self locking.  24KN/8KN load, aluminum.

Thanks to David Thompson who actually sent me one of these.