I highly recommend crystals manufactured by Performance Devices:

http://www.performancedevices.com/

They offer better-than-OEM quality crystals and can fit most any brand of receiver, including Hitec, Futaba, JR, and Airtronics.

Their crystals are good enough that Tower Hobbies had previously been selling their Airtronics and GWS compatible crystals.

If you have a question about finding a crystal for your particular receiver, just e-mail or call the company. They're very knowlegable about RC crystals. I own a dozen or so of their Futaba dual conversion crystals and have never experienced anything but perfect performance while using them.