Under the IRS rules, homeowners should obtain a copy of the tax credit eligibility certification from the manufacturer, installer or retailer when buying a qualifying product. You don't need to submit the certification to the IRS, but you should keep it on file with your other tax documents just in case any questions arise. 

If you know the brand of the roof you had installed you may be able to find the tax credit certification yourself online.  For an example check out Englert's Metal Roofing Tax Credit Certification PDF - located in the lower left corner of their roofing page.

If you don't know the roof manufacturer or your manufacturer does not have their tax credit certification statement posted online, I'd suggest contacting the installer. 

Wish you the best in tracking this down, if you run into any trouble let us know.  For anyone else looking at installing a tax credit qualifying product, I'd highly recommend having this certification in hand before beginning work.  Without it you run the risk of being sold a non-qualifying product and being stuck with the tax bill.