410 would probably be your best bet, as it has the best formability characteristics of all the stainless steel varieties and does not work harden too much. It is a little lacking in corrosion resistance though, so if they will be in a corrosive environment, you might want to try 316. 316 will be a lot tougher to bend and form, but it should work okay. If you have to weld them, forget about 410 and just go with 316. Be aware that 316 work hardens like crazy, so if you need to drill holes in your brackets, start drilling and push that bugger through as fast as you dare without breaking the bit. Another one to consider is 304, it is one of the 'general purpose' stainless steels, but like 316 it work hardens pretty quickly.
A good overview of the characteristics of stainless steels is available on McMaster-Carr's website, and I have included it here for your convenience.
When you need a corrosion-resistant metal, stainless steel is a practical choice. It contains chromium, which makes it more corrosion resistant than other types of steel. The designation 18-8 refers to stainless steel that has approximately 18% chromium and 8% nickel; it generally applies to types 301, 302, 303, and 304.
Hardening—Some types of stainless steel harden by cold working, and others can be heat treated. The 300 series hardens by cold working, many in the 400 series can be hardened by heat treating, and those designated “PH” can be heat treated by the process of precipitation hardening. All hardness values given below are in the unhardened condition.
Corrosion Resistance—An invisible film forms on the surface of stainless steel when it’s in contact with oxygen. This allows it to withstand damage from corrosives including many acids, bases, and detergents, as well as salt water.
Formability—Stainless steel is generally formable and bendable, but types that harden by cold working can require more force to bend than carbon steel.
Machinability—During machining, stainless steel can become gummy and stick to cutting tools, making it typically more difficult to machine than steel.
Weldability—Take care to clean stainless steel before and after welding operations. Contaminants, such as lubricants and particles from grinding tools, will reduce corrosion resistance at welded points.
Use the chart below to identify the best stainless steel for your application. Circles indicate that a majority of a material’s shapes and sizes meet the applicable rating. Yield strength is approximate and may vary based on size and shape.
Hardness
As hardness increases, metals become more wear resistant but they may be less malleable. The chart below shows hardness on different scales.
Finish
Stainless steel does not always have a shiny finish. In fact, stainless steel is available in a number of finishes, ranging from an unpolished, dull surface to a reflective mirror-like shine.
