We tend to buy old billhooks as they are often the best. Junk shops, stalls at real agricultural or wood shows are good places, although you sometimes can get them on e-bay.
Good makes are Elwell, that we like, Morrises which are still going in Devon and a few more.
I would always try before I buy, as I need a fairly light one. All of the different shapes are for different areas or uses.