Assuming you are from the NA:
#sna.gather would probably be the best bet, as there are a lot of clan people on there. You can also join and idle in the various clan chans and just "/amsg looking for a scrim."
Various clan channels from the NA:
#sm.soldat
#deth.soldat
#thewar
#demolitioncrew
#oc.soldat
I would recommend playing in gathers if you can't find a scrim. Use #sna.gather and #fractured for that.
Edit: Note that all of these clans are very good, but playing against good people is probably the best way to get better.