If you need a chunk of metal for a custom piece for your rig, what are the options? The best way to go is to buy a part that is close to what you need and then modify it, probably some sort of optomechanics. Alternatively, you can buy a chunk of metal and machine it from scratch yourself. McMaster-Carr is a classic place to get raw materials in US, and Fastenal has a smaller, but similar product line. Online Metals, Speedy Metals, and Metal Express are other places to check. RS is the first place I check in the UK (they’re listed under “Workshop Consumables”, and then “Engineering Materials”). Most of the time, the piece you can buy is woefully oversized for what you need. If you don’t have all the tools in your shop to do the rough shaping (cold saw, band saw, horizontal band saw, etc.), then ask a local workshop to do it for you. This sort of rough work should be dirt cheap with fast turnaround, and it saves you having to buy a whole new tool for your own workshop.