Is it possible to get them at one specific moment, or go to different periods and earn them in some way?
Yes. You can get good condition old series bills today without much issue. The older the more valuable though
You can easily counterfeit older bills, as technology has surpassed the contemporary anti-counterfeit techniques the bills were designed with. And as Doc has traveled into the future and retrieved technology from it, he likely has something akin to a perfect printer. And valuables to trade in the present day (much like Biff used the Almanac).
Notice Doc doesn't have any bills newer than modern day. He was simply preparing for any potential time travel, instead of acquiring it at the time period they came out.