To build supressors, you first must have a manufacturers FFL and then a Class 2 SOT Manufacturers license. Class 2 Mfg sell to class 3 dealers who sell to SOT...special occupational tax payers..AKA: Tax stampers.
If for your own use, in a permitted state, you may file an ATF Form-1 for permission to make your own supressor. Once approved and the $ 200 tax paid, you may then make your own supressor.
Hope that helps.
State Legal chart ?