I had purchased BestTrack's leveling kit when I bought my track but I had to modify the kit to compensate for my uneven basement floor. There's a floor drain and the floor is sloped towards the drain so I had to add some extensions to the feet in the middle of the room. I made them about 6" long so I can vacuum underneath while the track is up. The problem is when I take the track to a venue with a flat floor, I didn't want to have to readjust the feet for the flat floor and then have to relevel when I got home. So I made a second set of feet that are marked for location and pre-adjusted for a flat floor. Tried to keep it on the cheap so I used some 1/4-20 coupler nuts, jam nuts, and white plastic leveling feet. I needed 38 of the feet because I also placed a 1" x 1" x 14" aluminum angle at the halfway point of each straight section of track. The coupler nuts replace the outermost nuts at each track connector and stay on the track all the time. I put two jam nuts on each leveling foot so I can lock in the adjustment.
Coupler nuts were $0.17 each: https://www.mcmaster.com/#90264a435/=18b4909
White plastic feet were $1.68 for a 4 pack: https://www.mcmaster.com/#23015t82/=18b4drc
Jam nuts were $3.83 for a box of 100: https://www.mcmaster.com/#94846a029/=18b4ez6
I know BestTrack says you don't need the levelers if you're on a flat floor but I didn't want to risk marring the venue's floor and I don't use the padded binder clips to join the track, I drilled for thumb screws and wing nuts instead.