Most people just make up their own and copy it. Just think of all the information you will need. It doesn't have to be fancy. You can also make one on your computer where you can fill in the blanks. make two copies and you have one for the customer too.
I use pdfescape.com, because I like to be able to custom create my form with its own fillable form fields, plus I can email the forms and save them to my computer easily. Works great and it's free! Takes a sec to get the hang of it, but it's well worth it! Hth