autozone, napa, etc. any place has em. Waterproof grease is the way to go.
Don't grease too often, because every time you grease a little dirt gets in there. IMO, it's best to grease every so often enough that you see a little grease popping out of the seals. Then every now and then just clean the zerk off and give it two or three squirts of grease...not enough to make it start oozing out the seals. How often the grease is needed depends on how much you ride and in what conditions, so use your discretion, but try not to over-grease...it's almost as bad as not greasing at all!!
Also, while the a-arms need grease and all, the more important parts to grease, IMO, are the swingarm bearings, the axle bearings, and the front wheel bearings. You have to take things apart to grease these...make sure all of these are greased up good once a year and the bearings will all last longer!! The swingarm bearings love to dry up and everyone seems to neglect them.