Good 40mm grenades ?
 

There are quite a few brands/models out there, which one would you suggest buying for a M203? And where can I get them ?

Madbull nades are among the best. I haven't had any problems with mine (cycled maybe 20 times). I'm using vanaras POM nades as well and so far they're good. No issues so far but it's too soon to tell. Check airsoftparts.ca and in the retail section. sasksoft is selling the POMs.

The POM grenades will, at their worst req new valves (madbull ones fit) and a new oring. But even then it keeps things well under $20 for each grenade.

Madbull XM108HP, XM204XP or XMPB4. Can't go wrong with them. They are capable of up to 800psi on CO2. 900psi is possible but they become too sensitive and can be triggered by accident very easily. Also they can be powered down with propane for CQB to make it much safer.

Propane only nades are fine but severely lack in range.

Madbull for the win. I've tried several other types and found Madbull the best.