Be aware to be realistic with your expectations, although you always hear stories about bargains bought in thrift stores, garage sales and charity shops or treasures found in old garages or attics.
The truth is that the chances of finding something very valuable are not that high.
My own personal experience - which of course will not necessarily be the same as that of other people - is that it is incredibly unusual to find anything valuable and when you do it is very rarely for sale for pennies!
In fact only once have i found a bargain in this way which was worth considerably more than i paid for it - and in this case i actually paid $500 for the print (in its original frame), this was also several years ago so with inflation that would be a fair bit more than 500 in today's money.
However i believe value is about more than money, owning a genuinely old print that you get pleasure from is very worthwhile even if the print isn't worth much in cash terms!