I've seen sets of brake dies for arbor presses, but they seem expensive and fixed-length. Any one seen a 'cheap' source?
Alternatively, I could make my own if I can find press-brake dies in short lengths; any sources? Yeah, I know I could make my own dies, but that doesn't sound fun.
Check with machinery dealers in your area that sell hydraulic press brakes. Two types of brake dies are made, the normal "air bend" and the "coining" type. Coining dies produce a crisp bend and are my preference but take 2-3 times the tonnage of air dies. Normally you would have to shim the dies to produce even bends in use.
You should be able to get an off-cut as they are sold by the foot and come in 12' and longer (custom) lengths. Because they are special steel they are not cheap but they might have the odd pieces laying about.
If you are only doing a few pieces I would make my own dies or take the work to a commercial shop and have them do it properly. Most steel suppliers will shear and brake to your specs for a minor increase in price. Some shops have CNC Turret Punches that can cut, punch, and form smaller parts at high speed.
Hope that helps a little.