Usually they are grown from seeds. I would just pick a variety that is cold hardy and vigorous to use as rootstock.

You can go to this link to get an idea of varieties.

http://www.hort.purdue.edu/newcrop/morton/avocado_ars.html

I've used seed from regular grocery store fruit as my rootstock many times and they've all done fine. Graft it when the seedling is very young. When it is around a foot or so tall. Do a simple cleft graft, then tie with rubberbands and cover with a plastic bag. Place in the shade. After about a month cut a few slits in the plastic bag to get the plant uses to the humidity outside the bag. Remove the bag but do not touch the rubberbands. Let it fall off on it's own.

Last year I had three avocado volunteers in my compost pile, and I thought "What the heck? More grafting practice!" To my surprise, 2 of the 3 grafts took, and they are now both a foot tall.
Go for it!

Carla in Sac

Interesting. I have been reading on the net and go into the UC Riverside site where they talked about the different types and some that were better than others.

Fuerte type onto a Hass? Would Fuerte to Fuerte work or does it have to be a different type? A different fuerte tree for the union or could you use the same trees fruit to graft onto?

You could use the same type to graft onto.

With avocadoes a specific rootstock seems to be less important for a homeowner who is grafting a few things here and there. I have seen at a commercial nursery where they get hundreds of the same variety every year because that seed produces a rootstock that does very well in South Florida. The information you found regarding certain rootstocks that are better is not wrong it just applies more to a nursery or orchard and less to hobbyists.