Johnsonite makes hard rubber wedges exactly for this purpose. It's called the Johnsonite subfloor leveler system and it consists of strips of hard rubber all 4 feet long and 1 foot wide but with different thicknesses on each side, as follows:
LS-40 1/4 is 0 thickness on one side and 1/4 inch thick on the other.
LS-40 D is 0 thickness on one side and 3/8 inch thick on the other.
LS-40 E is 0 thickness on one side and 1/2 inch thick on the other.
LS-40 F is 0 thickness on one side and 1/8 inch thick on the other.
LS-40 G is 0 thickness on one side and 3/4 inch thick on the other.
So, imagine you want to run carpet up to your 3/4 inch thick travertine tile:
You could have an abrupt 1 foot wide transition by using an LS-40 E and have the top of the quarter inch thick carpet come flush with the top of the tile. You'd have to lay the tile, measure the carpet thickness and decide what transition to install beside the tile so that the carpet comes up flush with it.
Or, you could have an 6 foot wide transition by buying six LS-40 F's, and using from 1 to 5 shims of 1/8 inch thick material (like vinyl composition floor tiles) under each one.
And, anything in between.
Johnsonite is a manufacturer of rubber and vinyl flooring materials. They won't sell to you directly, but you can order their products from any flooring retailer in your area, or probably save a few bucks and order them online from places like this:
Subfloor Leveler System - Tools4Flooring.com