In my experience, if you're building a track with different elevations and other "technical" sections, you're better off to hunt down the 3/8" MDF.
It's easier to work with and to "twist" into place than 1/2".
Yes, cutting the slot at 1/4" leaves only 1/8", but that's all you need. Just be kinda careful while handling it.
As you've already found out, you can't get the 3/8" stuff from Home Depot or Lowe's. You need to go to a place like National Lumber. I got mine from them as there's one in the next town over from me. Had to order it as well.
If the track is going to be fairly flat (like Allan's), than by all means go for the 1/2" MDF that's readily available.
Here's a track made from 3/8" MDF. Note the elevation changes...and the Laguna Sega style drop turn you're looking for :D