Gallahad,
As far as spherical powders go, Accurate Arms offers many that are good for the 357 Magnum and 110-125 grain bullets. AA#2, AA#5, AA#7, and AA#9 can all be used. #9 will give you the most velocity, but it starts with a load of somewhere around 16 grains. #2 will give you about 200 fps less velocity, but the starting load is around 8 grains or about half the charge. The other 2 fall inbetween as could be expected.
That ought to get you started anyhow. Good-luck...BCB