Does anyone know of a source for 50s patterns?
FREE - One of the best resources for free and paid retro from the 50's through 70's is Tack-O-Rama. Plenty of clip-art, fonts and tiles for backgrounds
Dover publications are the long-time go-to source for historic imagery in reproducible formats:
Veer also has plenty of stock art collections including patterns:
Ten seconds of googleing found me this: