I made a similar adapter for my 8x10 Tachihara (allowing it to take 110mm Toyo boards), and couldn't find the slider bars at any of the usual small-parts vendors. Thus, I ended up making my own from aluminum bar stock. The only slightly tricky part was drilling overlapping holes for the diagonal slots with a fair amount of precision. From there, a bit of filing was all that was required.
(pardon the non-analog image at the following link)