While bead blasting is nice, it's not necessary. Fine sandpaper will work, as will a fine wire brush. But it requires more time and hand labor than bead blasting. There are numerous sources for phosphating kits on the internet, and as noted, Brownell's sells one. Most of the older gunsmithing manuals cover the procedures in detail, and also contain formulas for making your own phosphating solutions (but it's easier to buy them ready to use, and the results will probably be better). I thought Manganese phosphate was the method most used during WWII, but I don't know. Iron, Zinc, and Manganese phosphate solutions are all in use, and produce slightly different appearances.
By the way, the slightly greenish tint is due to a final dip into a sodium bichromate solution which imparts much improved corrosion resistance. That's an important part of the industrial procedure for application of phosphate finishes, but not usually performed by amateurs.
I will also add that application of one of the spray and bake Teflon coatings over a phosphated surface produces an exceptionally durable and corrosion-resistant finish. I have done that on many guns, and it's great. Phosphate is an ideal base for Teflon coatings.