Would this work for your girls? These I think are more for securing a car set, not used as regular seat belts. Would have thought a place like Babys R Us or Toys R us would have them or where ever there are a lot of car seats sold. 
http://www.dhgate.com/product/dhgate/163007544.html/?utm_source=pla&utm_medium=GMC&utm_campaign=houxiao0796&utm_term=163007544&f=bm%7c163007544%7c112007-InteriorAccessories%7cGMC%7cAdwords%7cpla%7chouxiao0796%7cUS%7c112007014-OtherInteriorAccessories%7cc%7c%7c&gclid=CLjTkKCF5roCFcti7AodvnoAEA
http://www.dhgate.com/product/latch-isofix-safety-seats-child-car-connector/163007545.html
Those belts are what normally come with the car seat. They attach to the LATCH anchor points that are usually built into the car. They won't do any good without the LATCH points.
If you feel you need to put a LATCH system in your car, and are willing to take the liability, you can make it work if you're handy.
Order a new LATCH retrofit kit from a dealership, or grab the pieces from the junkyard and modify it to fit your car. All cars built after 2002 are required to have LATCH. The factories made retrofit kits available for cars built on the same platform. Example: Toyota Camry was essentially the same car from 01 to 02, so for crash worthiness, there was no liability installing it. The engineering for the platform was already done.
I have no idea what car would make the best donor, but the key thing is the distance between the mounts. The lower anchors need to be a certain distance apart. Some of the ones I've seen came as a 1 piece bar already built to the correct size. Others simply use individual anchors that are attached to the floor pan and package tray. Be sure to use large reinforcements behind the anchors under the floor pan just like the factory seat belt anchors. Keep in mind, when using LATCH, these are the points that are going to take all the stress, and keep your kid buckled in the car.
Also note: most factory LATCH systems are rated at 40 lbs, including the seat. So if your child is approaching 40 lbs, you may just want to investigate the option of new seat belts vs installing all the hooks and plates etc.
The only reason I'm making these suggestions is because I'm assuming the kids won't be in the car often, most people are super careful driving their classics with kids in the car, and being belted with something is better than nothing.
