If you're in the US, just do a Google search for "resale certificate" and your state's name -- you should be able to find the form online pretty easily.
Bear in mind that a resale certificate, tax ID number, business/occupational licence etc. are all different documents. Some suppliers who ask for one of these will accept another, if you ask them (ie. if they want tax ID# and you have a resale certificate, some will accept that.)