How can I find joists in my heavily-textured garage ceiling?
Studfinders should work fine regardless of texture. Most have a deep scan mode for such situations, but I'd be surprised if that's even necessary. 

If that's really not working, look for clues. Your overhead door is probably suspended from ceiling joists. You may have an outlet or light boxes attached to them as well. You may find nails in exterior fascia indicating rafter tail location.

Most modern truss systems are 24" on center. Older hand-framed roofs are likely 16" on center. Once you find one you should have them all.