The three sizes of Pacemakers (2X3, 3X4, and 4X5) each had a specific size lens board, I'm afraid. The boards for the 3X4 are going to be rather difficult to find. It is possible some of the 4X5 Pacemaker boards listed on eBay are actually 3X4 size, and the seller doesn't realize the difference, as they are similar in size. But you would need to email the sellers for exact size of the board they are selling to know for certain. If you buy a 3X4 Pacemaker, you should try to get one with a board, even if you must pay a bit extra for a lens you don't necessarily want. This thread on graflex.org gives the dimensions of the 3X4 board and mention of making your own.