The only place I know of on that side is L&M. They are on 1097 heading east a few miles before you get to the lake and they don't open until 6:30 or 7:00 in the mornings. You can get minnows in New Waverly or on the east side of the lake on 1097. At the Citgo. Some of the marinas sell them but you may want to call first.
If you're coming from Tomball or from the west on 105, go to L&M. About a mile north of Montgomery, on 149, go west for two miles on 1097. It's on the left just after you cross the Little Lake Creek Bridge If you go early in the morning, tell them to dip out new minnows for you, they have a habit of leaving bags out overnight and selling them the next morning, resulting in dead minnows by the way you get to the hole.