I'm just curious because the binder I am looking into, (the 998 says 36-39 is medium) I am 38. So will it be safe purchasing the medium? Or do they run small?

Are you 38 according to the specific binder measuring formula?  If so then yes, I would get the Medium.  In my experience they don't run too small, but I'm not large-chested - others may have a different experience.

I'm between a x-small and a small. On their chart I measure 30.5. The small is 31-33 and the x-small is 28-30. Does anyone have an idea which one to get? I really want to get soon while I'm at home for the holidays and my previous binder was far too big. I don't want that problem again.

I think they might. I tried the 997, but that was way too tight. I couldn't get my shoulders through. 

Underworks are infamous for constantly changing their sizing. Out of the three major binder companies (Underworks, T-Kingdom and LesLoveBoat), they run largest because a) they were developed for cisguys with gynecomastia and b) they're headquartered in the USA, while T-Kingdom and LesLoveBoat are headquartered in Taiwan.

Thanks guys, I've decided to get the x-small with the idea that even if it is a little tight, it'll probably stretch a little with wear. I don't want to end up with two binders that are too big for me.