All you need do is check out the Canon M lens lineup. That's really all the lenses that are available for the M line. A few off brand but not much. Have you already made this purchase? If not I would urge you to check out the SL3. All your current lenses will work.
To my way of thinking it makes little sense to go to the M series. Especially when the R series is so much better and most likely going to obsolete the M series. Shortly!
I would recommend either the EF-S 10-22 IS USM, or the EF-S 10-18mm f/4.5-5.6 IS STM.
A lot depends on what you mean by 'wide-angle'. There are three lenses that would fall under this category:
The EF-M 11-22mm is has the widest Field of View, allows zooming obviously, and is an excellent lens. I have one and it is one of my personal favourites for wide-angle work
EF-M 22 and 32mm lenses are fixed focal-length units. They have the advantage of being able to gather more light, but are obviously not as wide as the 11-22.
That's about it in the native EOS MF line-up!
There are lots of possibilities, depending upon what you consider "wide" and need to shoot with the lens.
I have an EOS M5 and use a Canon EF-M 22mm f/2 STM with it.... moderate wide (comparable to 35mm lens on full frame) with autofocus. That's my "walk-around lens" for street photograph. It's an extremely compact lens and quite sharp. The only thing I don't like is the silly little "lens hood" Canon offers for it. I instead bought a generic screw-in, metal hood that nearly doubles the size of the lens, but the whole thing is still quite compact and the hood does it's job very well.
When I need even wider, I use a 12mm f/2.8 "Meike" lens (also sells under the Opteka brand). This is equiv. to approx. 20mm on full frame. It's a fully manual lens... both the focus and aperture control. But for my purposes, this is no problem. I tried using other fully manual lenses...less wide and short tele,... but ended up switching to autofocus and electronic aperture control with all of them. Of my four prime lenses that I use now on the M5, only this very wide 12mm lens is manual. There's such deep depth of field with a wide lens, especially when it's stopped down, manual focus isn't a problem for me.
If you opt for a prime lens (instead of a zoom), there are a number of similar fully manual lenses. In fact, searching at B&H Photo I found nineteen manual primes ranging from 10mm (FF = 16mm) to 25mm (FF = 40mm) from makers like Rokinon/Samyang, Meike/Opteka, TTArtisans, 7Artisans, Mitakon Zhongyi and Bower. These are mostly quite affordable and many are surprisingly good... if you don't mind the manual control (your camera has features like preview zoom and focus peaking to help with manual focus). These manual lenses have no electronic communication with the camera, so it doesn't know it's installed. Because of that, the camera needs to be set to "release shutter without lens" (by default it's set to not allow shutter to be fired when there is no lens installed, probably to protect the shutter).
There are only a few wide primes in EF-M mount with autofocus... the Canon EF-M 22mm mentioned above, a rather large Sigma 16mm f/1.4 (FF = 25mm) and a Viltrox 23mm f/1.4 (only sold in silver finish, FF = 35mm). All three are excellent lenses. In fact, the Sigma and Viltrox might be even better built than the Canon!
If you want a zoom with autofocus, well it's pretty easy... there's only one truly wide angle zoom with AF: Canon EF-M 11-22mm f/4-5.6 IS STM. By all reviews, this is an excellent lens, too. I haven't used it personally. Reasonably small and light, it also is the only wide angle I'm aware of with Image Stabilization. To keep it compact and fairly affordable, it uses a variable aperture and isn't particularly "fast". Some of the primes are two or more stops "faster". However, often large apertures aren't all that necessary for wide angle work. More often than not we're stopping this type of lens down to middle apertures anyway.
Finally, you probably got an EF-M 15-45mm f/3.5-6.3 IS STM kit lens with your camera, which also goes fairly wide (16mm on APS-C = 24mm on FF). If you didn't get this with your current camera, but want a 15-45mm, you can probably get it at a very fair price in kit with the new camera. .
You can shop wherever you like, of course (I bought my Canon 22mm f/2 "refurbished" from Canon USA, other lenses from Adorama, B&H and off eBay), however the B&H Photo website lists virtually every possibility and has various filters that can help you narrow down the selection: EF-M mount lenses at B&H Photo.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.