Ask your doctor's office or the medical facility in question if it has a medical release form that you can use. Doctor's offices almost always have one that you can fill out and bring back to them. This might save you a lot of time searching for a generic medical release form elsewhere, and it will contain all the specific information that they want for the release.
Check online legal form sites such as LegalZoom and FindLaw, which are good places to look for generic medical release forms and other legal forms. You can print them immediately, but you will still need to verify that they will be acceptable to the medical facility or doctor's office that you will be submitting them to, so it's best to find out ahead of time what information needs to be on the forms to satisfy the release requirements.
Visit office supply stores, as they usually stock pre-made legal document templates, such as a generic medical release form. They usually come in packets with multiple forms inside and are sorted in the aisle by category and usually cost less than $20.