I'd recommend you buy a bead roller and a sheet or two of cold rolled mild steel and start by practicing to get the technique down. There's a lot of videos on youtube showing various bead rolling techniques.
The only practical way to create the diamond pattern in a brake is via the use of a press brake with a set of cross braking dies. Its near impossible to maintain consistent bend angles in even a heavy leaf brake as the operation progresses, due to the fact that you have to alternate the direction of bends to keep the sheet somewhat flat. Attempting to just progress across the sheet in one direction results in rolling the sheet into a curve. OTOH, alternating directions in a leaf brake means each successive bend is crossing more and more previous bends which will not clamp flat. The result being the appearance of the bends changes as you progress across the sheet. A press brake has the power to flatten the intersections of previous bends and maintain a uniform appearance.
Even if you were to buy a piece of pre-formed sheet, you'd still need a bead roller to deal with the edges.
Having seen firewalls done both ways, either with a full quilted sheet or with panels created with a beaded edge and then bead rolled with the diamond pattern, I'm of the opinion that the panels make a much nicer looking finished job. The full sheet approach just ends up looking like too much of a good thing. I've spent 50 years in the metal working trades and have a well equipped industrial level sheet metal shop. I can tell you from experience that the panel approach is also a much easier way to end up with a good overall result. To be clear, the panels are created as a part of the sheet itself and not separate pieces to be applied to the sheet.
I'd also recommend making a firewall "cover" as opposed to attempting to make an actual replacement firewall. A cover can be made from 22 or 24 gauge metal which is orders of magnitude easier to work with than the 16 ga that would be needed for the typical firewall replacement. The lighter material also makes it possible to use one of the lightweight leaf brakes available from Harbor Freight to put any necessary bends in the metal since the typical firewall isn't a flat panel unless its on something like a T bucket.