I've used the Respro City Mask with moderate success. Hardest thing is keeping the seal around your nose and under your chin while under the helmet, and this one worked decently.
I've also been able to fit a disposable N95 mask with my helmet, but I have a modular helmet so it's got a little extra chin room
http://respro.com/store/product/city-mask
Side note: It gets hot as hell with the mask on.