Whether there is a practice day before the race depends on the track. This year we had or will have Saturday practice at Round 1 (Hwy 65MX), Round 2 at Sweeney MX and Round 8 at RiverBottom MX. Round 7 is a Saturday night race at HLR Motorsports so there isn't practice the day before, and Rounds 3-6 are at Midway MX which has stock car races on Saturday nights. If you contact the track owner, he often will allow you to practice early on Saturday but you have to be out before the gate opens for the stock car races. However at Midway MX we run long practices before the race, so there's plenty of time to learn the track. In Missouri we have an arenacross series put on by Motorhead Events out of Springfield that always includes a pitbike class among the regular big bike classes. There's a series in Stillwater, OK that also runs pitbike classes. Unfortunately most of the tracks we used to have in South and Southwest Missouri have closed down due to the poor economy, but we have gotten it to where most Missouri tracks will allow pitbikes to run on their regular practice days. Cabool was one that wouldn't have been too bad a drive for you. But it's shut down.
River Bottom MX is at Lake of the Ozarks, and the track is actually open for practice every day except when there is a race. There's a campground at the track which is alongside the Osage River, plus you're at the Lake of the Ozarks (literally, not just a few miles from it but at it) with all the stuff the tourist area has to offer. So it's worth the trip.