Pine 4 inch planks needed 7x5 ft

We recently uncovered plywood in hallway to find that underneath, part of subfloor consists of old pine planks and part (approx 7×5 ft) is plywood. We would like to sand and finish the planks, as we’ve done elsewhere in the house. However where do you find old pine 4 inch planks? Does anyone have any they would like to get rid of (or sell)….Is it possible to use new pine planks and refinish them. We hope to do this within the next 10 days while Father in Law is visiting to supervise the work during the day, so anyone with planks immediately available would be much appreciated ! I can also be reached at crownheights2007 @gmail.com Thanks !