You can buy a string jig from many vendors such as 3 Rivers. You can also make your own pretty easily. Of course, a string board only makes measuring the strands of a Flemish string easy. You don't need one to make strings.
B-50, serving material and a serving tool can all be purchased at the same place you get a string board. Two 1/4 lb spools of B-50 will last for quite a few strings. Get two colors that go together and you're good to go. You'll also want string wax. You can buy it or just use beeswax from your local hobby or sewing supply store.
Guy