For simple bangles, all you need is a bangel mandrel, either round or oval. Steel is expensive, hardwood much cheaper. I got by for a very long time with an old table leg and a baseball bat.... You can work with round, D or rectangular wire, or with sheet.

If you want to create anticlastic shaping (where the outer curve is perpendicular to the main curve) you need a stake. You can either get a steel sinusoidal (very pricey if it's for occasional use) or a delrin one - such as Peter's. I'm delighted with my delrin one as it allows me to use a regular cross pein hammer, rather than having to source a nylon version from the US.

There was a nice tutorial in Art Jewelry Magazine a while ago - covering both reticulation and anticlastic raising - it might be worth getting a back issue for that - it's barely more than buying the pdf version.

Art Jewelry Magazine - Issues