For cages its best to buy one from a company that has been making cages for some time and have a proven design. I mean yeah you can get some local fab guy to weld up a bunch of tubing, but they dont have experience in building them for a specific bike it may not do what you want it too. And can do little to protect your bike or even worse cause more damage to your frame because of bad design, welds, joints, or materials used.
Subframes on the 03/04 636 with a good 12 bar and a subcage they can take a pretty good amount of abuse before you trash it. I ran stock subframes for a long time, then I built a custom one out of solid aluminum. Some companies make steel ones but they are heavy, and change the way the bike feels so I never really liked them. Same deal though you can get a local guy to make you one, bu again its better IMO to buy from a company that already has been making them and have a proven design. And they usually stand behind the product, if you manage to break it they will usually fix or replace it
If you are still looking for a stock subframe my friend is parting out his 03/04 he has 2 for sale