Hey Jonah, when hammocking some people find that laying on their bag makes the insulation compress, and thus not work. That's why we use pads or underquilts.

You've got to think about under insulation and top insulation.
Under: pad or quilt.
if pad, think with wings to keep your shoulders warm.
if quilt, 1/2, 3/4 or full length (shorter ones augmented with a small pad for your feet)
also: down or synthetic? if synthetic there are several types. DIY or bought? (search for PLUQ- poncho liner UQ)

Top quilt: happy with your existing sleeping bag? or get a top quilt (as your underside is taken care of by the pad or quilt.

Lots of vendors, lots of options. Different types for different preferences. I have JRB (Jacks R Better) quilts and they are nice, some folk like Hammock Gear, Leigh Lo, Arrow-head (synthetic option) and WAY WAY more out there. Research- a lot of these guys are forum users and old friends, so I don't really feel comfortable saying "this one is the best one" - find one that works for you.