Your state of residence presents an issue. I wouldn't be the least bit surprised to find that Massachusetts limits how much powder a dealer can keep on hand, probably something totally nuts like 15 lbs. I would suggest working up a list of every firearms retailer in New Hampshire and spending an afternoon on the telephone calling them all. That is probably the nearest potential source for Magnum class powders in your area.
BTW, a fairly complete list of powders suitable for 357 Magnum in order of preference is as follows. Alliant 2400, Hodgdon H110 (also known as Winchester 296), Vihtavouri N110, Accurate #9, Vihtavouri N105, IMR 4227 and SR4759, and Alliant Power Pro 300-MP. Powders more suited to 44 magnum that may work for the 357 in a pinch - Accurate 4100, Hodgdon Lilgun, Accurate 5744.
Of all the above the powders those that are IMO are most suitable for a 6 inch revolver are Accurate #9 and Vihtavouri N105, and perhaps 2400. Hodgdon H110, IMR 4227, and others on that list will work for Magnum loads but I believe they are a bit too slow for use in a Handgun load and are best suited for use in Rifle loads. As for why, Flame Cutting and lots of muzzle flash with a trademark BAAHHOOOOM on each shot lets everyone know you are using a slow powder in a short barrel. Yeah, with H110 a 6 inch barrel is short.
BTW, I have 2 rifles in 357 Magnum and for that use H110 is a real favorite. Because even a "mild" charge of 14.8 grains will drive a 158 grain bullet to 1615 fps and a "moderate" charge of 17.5 grains will drive a 140 grain bullet to 1875 fps. My experience has shown me that slow powders really "wake up" in a rifle when in a handgun what you get is a lot of drama without much gain at all in velocity. It's why my favorite powder for handgun loads is Accurate #9, it's clean, accurate, and you aren't wasting what appears to be 1/2 the powder charge producing a 10 foot flame out of the muzzle.
