Where can I purchase Gap master, a caulk that is made for gaps in a hardwood floor? I have read it is the best, but I can't find it in any store. Does anyone know where I can buy it or something that is similar? These are a pine wideplank hardwood floor, that was installed, but they nailed them down, and they did not butt them up correctly, so I have gaps. The floor is a finished pine floor.

Iam no flooring expert, but I think caulk may be a bad idea. Have you lived with the install for an entire year? Its possible the gaps will close up as the humidity and seasons change. If the moisture content of the wood was not acclimated properly before installing, then the gaps may be more permanent. The problem with caulk is that an exact color match will be very difficult and even if you do match it, it will change as it collects dust and dirt.