The 'normal' blue product is Tordon RTU - like treeman 75 said. It is about $15 per quart. It can also kill other plants - especially tulip-poplar - in the area, so be careful. Only use a little around the outer edge. Most farm-type stores carry it around here. It is made by Dow, so if the store deals in Dow, they should have it or be able to get it easily. The chemicals in Tordon are picloram and 2, 4-D so you may find generics with similar chemicals. Pathway is similar and is also made by Dow.
I have seen Garlon 4 in smaller containers. The active chemical is triclopyr. I have definately seen triclopyr in smaller containers under different names.
Neither accelerate decay, but they do kill the root.