I know nothing about B&J lensboards, but if they're just flat pieces of wood then you might as well make your own. If you're handy with tools you can make any kind of lensboard you want to, but the flat wooden ones are easiest.
I use hobby plywood for all of my lensboards. For flat boards, just measure the thickness of the board, find a piece of hobby plywood at a hobby shop or art supply store that's the same thickness and at least as wide as your lensboard, trace your existing board on the plywood, and cut it out. To make the center hole, you can use a drill to punch a bunch of holes around the perimeter of the circle then use a box cutter (or even a pocket knife!) to remove the wood between the holes you drill...voila, a lensboard with a hole that matches your current one. You can clean up rough edges easily with sandpaper.
If you have a Dremel-type rotary tool or a jigsaw at your disposal, things get even easier.
Even Linhof boards (those small things with all of the funky angles and cut-outs) and Cambo boards (with rounded corners and the light trap on the back) can be made with just a little thought. I can't tell you the last time I actually paid for a lensboard...it takes me 10 minutes and about $1 worth of wood to knock out anything I need with a Dremel tool and a jigsaw.
Again, if your board is just a simple square of wood, making it yourself is a no-brainer. If it's more complicated, then it's up to you to decide if you feel comfortable making a new one...but in my experience lensboards are the single most overpriced part of a view camera, given how easy they are to make.