I have purchased accessories from Just4Electronics. Good prices but shipping rather steep, and needed a couple of follow-up calls to make sure items shipped.
If you buy from HB, do a phone order vs online. If you buy from online, some sort of reverse auction takes place, and I have had delays with that.
I think (but not 100%sure) the GR16 will work. But if I was buying new, I'd just get the GR50. The GR50 will get satalite lock quicker and will be more accurate.