Cheap way is probably to buy some solar film from astronomical kit suppliers. Then cut them into a suitable filter size for the front of whatever lens you use. Pre-made ones are more expensive. I'd go for something that is rated as safe for optical viewing, cos if things go wrong you don't want to be on the receiving end of the sun! They reflect most of the sunlight and leave enough to get through to image.
Thinking more, that would be most use for sunspot watching. I'm not sure if there's other detail that can be picked up easily.
Size wise it is similar to the moon, so if you want to fill the frame you're looking at 1000mm+ actual focal length on crop sensor DSLRs.
_________________Canon DSLRs: 7D2, 7D1, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 85/1.8, 135/2+SF, 70-300L, 100-400L, 100L, MP-E 65, EF-S 10-18, 15-85
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 50/1.4A, 150 macro, 120-300 f/2.8 OS
Compacts: Sony HX9V, Fuji X100.