What enlarger do you have? There are plenty of cheap used 39mm lens boards available for Beseler 45 enlargers. A 150mm lens will need a board with a 50mm hole. It is less common used, but not that hard to find. Both the 39mm and 50mm Beseler boards are available new, but they are pricey.
I have a Beseler MCRX 45.. I have several boards including a board that came from a 150mm lens so i wont have any issues with having a board.