My roommate backed out of our co-signed lease two weeks before the lease's commencement. Can she force me to find or accept a subletter or is she still liable to pay her half the rent?

It is likely that the two of you are responsible for the rent "jointly and severally". If you cannot afford to pay the rent by yourself I'd talk to the landlord about getting out of it... unless you do know someone who would take her place (and in that case you'd also need the landlord's permission).