I recently went to Good Will and spent $80 on stamps. They are Stampin Up brands, which is really good quality. They were fairly priced at between $5 and $10 per set depending on the size. My haul that day was easily worth three times the actual value, and they were all brand new.
Yesterday I found some amazing stamps at a yard sale. Again all brand new Stamp in Up. They were from a Stampin Up demonstrator who was paring down her stash.
Other great places are Facebook destash pages, there are many to choose from. The only bummer there us you usually have to pay shipping.
Of course many of the online stamp stores have sales from time to time. If you follow them on Instagram or Facebook you will know when the sales are on.