I almost always get my minnows from the star mart but they close at 9 Sunday-Thursday and 10 Friday and Saturday. Not sure about the marina hours. Pretty much every marina on the lake has minnows but be ready to catch mostly White bass at least this has been my recent experience.
It really depends on what ramp you are using. Star Mart minnows have not been the best lately especially later in the day if they are pre-bagged. The best place in my opinion is the little store on 515 just east of the TM repair shop. East from there the Phillips 66 (if it still is a Phillips) is pricey. I have not been to the Valero at 154 and 515 lately but it was good last time.