Would mulch be good if I were to buy it this fall and save till next spring or would it get moldy in the bags. I was just thinking if I could get a good deal and it would keep I may do this.
It depends on many factors. High quality mulch should not get moldy, especially if stored properly. You also have to make sure the mulch is packaged in a bag with holes to allow for good circulation. All said, it is unclear that the question can be answered definitely.
What I have noticed though, at least here in California, mulch is very cheap, and deals come out twice a year, in Fall and Spring, obviously, the former meant to help you mulch around the garden before winter, and the latter for when you start actively planting in Spring.
Depending on where you live, this might be the case with you as well. You could talk to your local garden supply stores to verify this, and instead of worrying about preserving the mulch over the winter, you could just buy the mulch twice a year. This is more or less what I normally do.
Yes it will still be good in the spring. However, it will be 6 months more decomposed, and probably last 6 months less. (If it is frozen most of the winter where you live this will be less of an issue.) That is not a bad thing though, my local municipality actually charges more for aged mulch!