Sporty's sells tie down kits as you describe. You can also get similar
ones in your hardware store in the section with tie down straps for
trailers although the Sporty's ones are a bit nicer.

One problem I've seen people have with them is that they don't take well
to being run over by a fuel truck if you leave them on the ground while
you are out flying. The buckles tend to squish out of shape.

Your local hardware store will also have other things you can use to
make some nice tie downs that are the proper length. My portable kit is
made out of good thick braided ropes of a distinctive color (so they
won't walk off) from Lowes with carabeners from the local kayak shop on
one end. I can use these to clip into the metal hooks in on the ramp at
many airports. Also, when I park at an airport that has too short of
ropes or ropes in ratty condition, I'll tie a loop in the existing rope
and hook one of my carbeners into the loop to use my rope as an extension.

Helen