you will have to make one- never seen one for sale.
However, they are not difficult- even i have managed to make one! Bit of gauge plate/flat ground stock. Use the existing as a template. Hacksaw or jigsaw with metal cutting blade to cut it. Dremel/file etc for tidying it up. Drill a hole or two and Bobs your uncle!
The more difficult part will be thinking how you are going to guard the blade. There are a number of options, depending what you are planning to cut. Dont skip this stage! If you are struggling, ask on the forum and I am sure you will get plenty of responses.