Here is the catch I often see. You can buy an already trained weight puller, however sometimes the dog will not work for you as well as it did the ones who trained it.
My recommendation is either buy a SEASONED dog with multiple titles that is automatic, (will still not pull as well usually) or buy a pup.
I would go with the pup and learn the breed and the sport together. You can start training a pup at 7-9 months old and can be competing within the year. This gives you a chance to bond and build a relationship with the dog. Much of winning in weight pull comes down to the dogs heart, not strength. Get a dog that loves you and wants to work for you, it works out better in the end.
"Life is like a grindstone. Whether it grinds you down or polishes you up depends on what you are made of."
But a Constitution of Government once changed from Freedom, can never be restored. Liberty, once lost, is lost forever. John Adams