Do we know how long ago it was that they fought with Negan? If a lot of time had passed, then they might have had time to find new weapons. They certainly had enough time to establish their new settlement. Also, they mentioned that most or all of their men died while fighting the Saviors. It could be reasonably assumed that Negan had not yet taken their guns before the fighting started. In that case, the survivors could have escaped with some guns still in hand.