For the longest time we tried finding a solution.
Once we realized the industry referred to them as "Water Dam" it proved a cinch to find. Just google water dam for wheelchair shower. You'll see numerous styles from various distributors. Ours cost approximately $100 the material was cut using a pair of scissors, it is collapsible so I can roll over it with my wheelchair without a problem . Glued to the bottom of the shower with epoxy glue I must say it has never leaked since it's installation.