another good place to look for raw materials is ebay. it's often cheaper than specialised retailers, if you just need one relatively small piece.
reg. your questions how to machine it:
Sounds like you don't have access to professional metal working machinery.
In case you do, a mill and a bending bench would be all you need.
If not, everything needed can be made with common hand tools, i.e., a power drill w/ various drill bit sizes, some files (to create the slots from a series of holes), a vice, some clamps, some pieces of scrap wood, a saw, .... and some common tools.
You should have the basic skills though, and I recommend to first try on some scrap piece....
I agree with Bob3 that working on SS with such tools is difficult. Not impossible, but really a bear
All in all, I suggest you only take this on if you want to do it for the sake of fun making your own stuff.
If you consider making one to save money, I recommend looking at low-price online places like Leisure Pro (they sell the OMS Al backplate for some 65.-), or
www.hammerheadscuba.com. They have nice low price SS and AL plates, that are unfinished. S you would still need to do some work to deburr it, but all the hard work is done.
This way, you will get a professionally made high quality item that will last for years. It's not more expensive if you don't have access to free scrap materials to use, consider value of your time, and may even need to buy some tools first.
d-s-f