As PicCircusDesigns said you will need from any supplier or retailer that you get the base clothing from a GCC, COC, or test results that have at least the lead levels for the non-exempt portions of the clothing reported to you in PPM. Most fabrics are exempt, but any buttons, zippers, snaps, grommets for lacing, any plastic or metal based additions will require you to keep copies of the test results or other paper work on file and matchable to a specific product that you sell. Also suspect are items that have screen printing instead of dying to produce the patterns on the fabric since the CPSC lumps most of the inks used to do this process as a surfacing coating akin to paint, which has stricter limits and more types of tests than other children's products.
Though it's required for manufacturers, importers, and retailers to have on file the test results there is no portion of this law that requires them to make the test results, GCCs, or COCs available to the public. It's doubtful that a major retailers like Old Navy or The Childrens Place are willing to to part with the required information to what they would consider a re-seller. Even companies like Gerber that acknowledge many of their products are used for embellished resale by crafters are reluctant to provide the required info unless you are ordering "factory direct", in which case you would in effect become an authorized reseller of their products. Basically even if you did nothing to the product in the way of embellishment you would still need to have the testing paperwork on file the same as a regular retailer would keep at their corporate office. Under the requirements of the CPSIA, in the case of being an authorized dealer, the manufacture is required to include the neccessary paperwork with each shipment.