Buy the RocketShip Games MMX launcher. Best one out there. Makes it simple to launch MMX rockets. If they ever sell more MMX rocket kits, their kits are fun builds as well.
RocketShip Games seems to be a little less expensive than other dealers on MMX motors. Recently got some for $9 when everyone else is at $12/6 pack.
Ollie-oop Rocketry has some nice MMX and 13mm downscales. Love the BT-50 based Phoenix downscale as well. Toby has some other items, such as a MMX motor adapter for BT-5 tubes, and a 2-motor MMX cluster for BT-20 tubes. Also sells basswood fins and centering rings for the BT-5 Der Red Max kit.
FliKits and Semroc also have nice MMX kits. Love the MX Cherokee kit. eRockets has the best collection of parts (tubes, lugs, centering rings, nosecones etc.) if you want to build your own MMX rockets.
Lately, I’ve started taking a kit and reducing the decals and fin patterns so I can make downscale versions for MMX. Did Citation Patriot series (BT-2, BT-3, BT-5, BT-20, BT-60), Centuri Hornet. Now starting on a set of Estes EAC Vipers (BT-5 MMX, BT-20 13mm, BT-50 18mm). I want to make scaled series of kits, so I have one rocket each for MMX, 13mm, 18mm and maybe 24mm as well. Use Krylon Crystal clear to seal your inkjet decal sheets. I put all my different decal sets (e.g. BT-50, BT-20, BT-5, BT-3) on a single sheet, print and seal them. Downscaling kits you already have opens a whole new door for MMX builds. They are addictive. I started building MMX last October and I have 16 kits completed, with 2 under construction right now. You can build and finish a kit in a weekend.