Johnsonite has a "subfloor leveling system" which consists of 12 inch wide 4 foot long strips of hard rubber that taper in thickness across their width. You can buy strips that go from 0 thickness (effectively) to 1/8 or 1/4 inch over 12 inches. If you want to go from 0 to 1/4 inch over a 6 inch width, you'd have to buy two strips and glue them together.

Essentially, you glue the transition strip down to your underlayment, and then glue your flooring down over the transition strip.

Johnsonite Wall Base, Finishes & Accessories Finishing Accessories Subfloor Leveler System

Any place that sells carpet, tile and hardwood would be able to order the materials from Johnsonite for you.