Don't fucking buy Mira products; they're overpriced Czech surplus. The OM-90 is the Czech designation for Mira's CM-7M, and you can get them for $90 on eBay, vs. the $450 or so that Mira charges.

Honestly, get yourself an M-40 that's in good shape, buy a 40mm-to-3M-bayonet adapter, use some 3M P-100 cartridges (you can get them for about $2/ea. when you buy in bulk). P-100 will filter out all particulates--even finer particulates than a HEPA filter or N-95--and will also filter out water- and oil-based aerosols. That covers all riot-control agents, because CN, CS, and OC are all aerosols, and not true gasses. Almost any surplus gas mask that's been made in the last ~30 years, seals, and uses a 40mm NATO cartridge is going to be just fine for riot-control agents.

If you are really worried about a P-100 filter being insufficient, get a few new old-stock NBC filters. NBC (nuclear, biological, chemical) filters were phased out about 15-20 years ago in favor of CBRN (chemical, biological, radiological, nuclear) filters, but if they haven't been used, they'll be absolutely okay for riot agents.

If you wear glasses--and many people do--get a spectacle insert for whatever mask you have. Glasses break the seal, and while you will probably be fine with contacts in, that's not necessarily a good risk to take. Supposedly the Kris Optical Mag-1 glasses can be worn under a mask without compromising the seal, but I can't personally attest to that. I think that they're one of the older styles of BCGs that were issued, but I'm not sure.

Oh, and get GOOD hearing protection. Consider high-end electronic ear muffs, and wearing ear plugs under them. Some places are using LRADs to disperse crowds, and those can wreck your hearing.