my boot zipper used to always break & i brought them into a show where they put in a heavy duty zipper!! they haven't broke again since
you could have the zipper taken out, elastic added either side (roughly an 1-2 inches of elastic) and then put zip back. that way there is more room in the back of your boot :)
Definitely take it to a shop to get fixed !!
You can take it to a reputable cobbler to get replaced, look for one of those old time shops where they do a lot of buisness, not necessarily your tack store! Sometimes there is a discount if you get both replaced at the same time. If it keeps breaking in the same spot you may be better off with a different style of boot...