Design inspiration

I've been stewing for the past week about fabric design, as in how the heck do I come up with my own?  I definitely know what I like, but there are just so many ways to go when I consider the prospect of making up something from scratch.  I have a good friend who's been running a vintage thrift shop here in Chapel Hill, Time After TIme, for the last 30 years.  When I told her about Spoonflower, she told me about a customer of hers, a fabric designer, who used to come in every few months or so to buy up enormous stacks of '30's era print dresses.  The condition of the dress didn't matter--holes, tears, and armpit stains were all fine.  This customer was buying them to copy their prints.  Huh.  Research on the whole issue of vintage print copyright has led me to understand that this is a pretty common practice.  Amy Butler did it, right? 

I do love me some vintage prints and, as friend to someone who can give me access to LOTS of them, I could have all the inspiration I needed pretty easily.  But is this a legitimate thing to do?  I still can't decide...