I get these splitters from surplusshed or surplus-optics.com. They are nomimally 15mm on each edge. I have tested several of these from surplus shed and they work just fine (will test surplus-optics soon). They are a little dirty but will give you excellent interferograms. Guaranteed to be able to get better than 1/100 wave accuracy on testing your mirror using a Bath Interferometer. Note that these vary in width by up to 1mm so you will be getting a mount/holder with the part even if you order this part by itself.