Some recorders/mixers, i.e. many from Sound Devices, have M/S decoding built into the unit. Granted, I believe you're actually recording the decoded stereo signal using that functionality, but you can always derive the M/S signal again in your DAW if you like.
You could also look around for one of AEA's MS38 Lite headphone amp. It was a very compact belt-pack decoder. They're discontinued now, but you might be able to find one used somewhere......just don't ask me where.
Here's a link to their page showing their legacy (discontinued) M/S products.
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To add to what @Shaun said, the Sound Devices 7 series recorders allow you to either record the decoded M/S signal or record the separated M/S signal to each channel while monitoring through the headphones the decoded signal.
Also, the Zoom H4n allows you to record the decoded M/S signal through the external XLR inputs. I've used this when with my stereo shotgun (sennheiser 418s) while traveling or recording in environments where I'd risk damage to a $300 recorder rather than a $2000 one.