"How can I obtain a medical proxy form for my grandmother I have a power of attorney but not for medical."
If your grandmother has any religious affiliation you should contact a religious organization of her faith on her behalf to learn more about what issues should be incorporated into her living will.
As for "form" documents you requested, standard simple forms on the Internet have created huge problems in many instances and you really need to be careful. Many organizations sell or give away forms, but you're much better off going to an estate planning specialist and getting documents tailored to your grandmother's needs and wishes. For additional background see the book Shenkman and Klein, "Living Wills: Assuring Your End of Life Decisions are Respected" which also has sample forms. You can obtain this from www.amazon.com. While it has sample forms, again, protect yourself properly by hiring an estate planning specialist and getting it done right.
Finally, be sure to let your grandmother voice her wishes. The decisions are very personal. Often, family are surprised by some of the wishes voiced.