I too have the Brother PE 770. It says up to a 5x7 hoop, I also have a 5x12 hoop I use.
I bought mine on Amazon from this seller.
http://www.amazon.com/Brother-Embroi...brother+pe+770
Great customer service by the way. I called a couple of times to ask them questions. And they answered each on politely and very professionally. I am still using the box of pre-wound bobbins as well as the box of assorted colors of thread.
I also bought this hoop set too:
http://www.amazon.com/Elipse-Embroid...W4096JE5M9J1M3
Now I have 2 of every hoop except the 5x12. I love it especially if I am doing several of the same embroidered designs on towels for example.