I picked up a 500 foot spool of #14 THHN stranded insulated wire at Home Depot for under $40. Because my radial installation is going to be temporary, for tests and measurements, I preferred the stranded wire to solid. But I plan to use a ring of #12 solid at the base of the antenna to solder the radials to.
My initial plan is to go with about 16 radials and make them about 25 feet long. This is not as long as I would like, but it's what will fit on the piece of property I have to work with.