You have several commercial options, although it depends on your budget...
1) Simple OD cotton t-shirt. Various vendors. Your best bet if you need this on the cheap.
2) FR shirts: Various types, most use modacrylic, vendors include Danskin, Potomac Field Gear, XGO etc. DriFire tends to be the most widely worn/available in Canada. Modacrylic, some vendors claim, releases noxious fumes on ignition...
3) Wool shirts: I only know of a few in merino wool, and mostly in foliage green/coyote; you'll have to research for OD green but it is a good option if you are not allergic...
3) Cordura Baselayer. My current personal pick. Not FR but no-melt no drip, feels like cotton and breathes (the key, as I find Potomac, and to an extent DriFire, traps heat) and cheaper (TacWear in Toronto is a local source; USCAV in the States is clearing these things out at $15 each)
There are even more out there (CarbonX, Massif, Arcteryx LEAF had a baselayer in "spruce" etc), but I think given cost and availability cotton and the Baselayer are your best bets. YMMV.