Trigger Shift equivalent to SRAM MRX 21 speed on Btwin Rockrider 5.0
The MRX twist shifters are 3x7 Shimano-compatible shifters, so you can use any 3x7 Shimano shifter set from the mountain-type group. 

Unfortunately, the only trigger shifters which are 3x7 Shimano compatible still made are from the Tourney line, such as the Shimano Tourney ST-EF40, which are of rather low quality. You may be able to find an old Shimano Acera or Alivio or Altus 3x7 shifter set if you look on Ebay or something. 

I had a 3x7 drivetrain on my commuter, and ended up swapping the rear freewheel (yes, freewheel and not a cassette) with a 8 speed freewheel, and getting some Shimano Alivio 3x8 shifters which were easy enough to find and of good quality. I wasn't able to get the Tourney shifters to give good shifting performance, and I don't like twist shifters (for the 3x7 drive trains, I suspect SRAM's Shimano-compatible twist shifters are the best of the indexed shifters which are still on the market in terms of reliability; they're all cheap pieces of junk these days). 

Another alternative is to use a friction shifter. Very cheap to buy, though you just have to get used to shifting into the right position by feel. This is what I'd likely have done if I didn't upgrade to 8 speed.