That depends. How absorbant do you want them to be? How thick can you stand them to be? Do you want them to be full protected, ie. around the sides as well for sleeping on your side or just front down through the crotch and back? You also need to consider PUL or other type of covering pants to guard against leaks. Keep that in mind when ordering. You need to factor in laundering cycles. How often will you be able to get to wash them? How long will one last before you have to change will dictate how many you need in one day and that will dictate how much laundry you need to do.
Consider those questions. I am sure others will have more input for you as well. As far as some places to shop. Here are some that I have found. You can also do a google search for Adult Cloth Diapers and come up with a pretty good list.
Angel Fluff Diaper Company -
http://www.angelfluff.com - has a pretty good supply of flat and prefolds NOT pull ons but also has plastic pants (definite need)
Adult Cloth Diaper -
http://www.adultclothdiaper.com - has a good supply including pull ons. They also have sections about cloth diapers and about diaper washing and folding and such. Also about plastic pant care. Good stuff there. Learned a lot at that site. It is actually from that site that I found this one.
And there is another one that I used to know of that I cannot remember right now. If I recall it I will come back and post it another time.
I hope this helps a bit. Good luck.