Does anyone know if there is a child's car seat kit that can be installed in an older car with only lap belts?

My husband has a 1986 Buick LeSabre and I can not get my children's car seats in it. There are only lap belts, once I pull the lap belt out, it locks so I can not tighten it. He heard there are kits you can buy or get installed that will allow us to put our car seats in it. Anyone hear of this?