A lot of people have recommended Power On Cycling as a source for entry assistance parts. The owner has been active in helping veteran rehabilitation for years. I bought stuff from him in the past and was always happy with my purchases. Here is a picture showing a set on a Catrike (model?) for $95. That's a lot less than the other sellers want.
https://www.poweroncycling.com/ https://www.poweroncycling.com/produ...t-bars-839.htm
The other source is Utah Trikes where they do a lot of custom fabrication and modifications to trikes. The cost is about the same as what you get from Terracycle.
https://www.utahtrikes.com/PROD-11618467.html
Terracycle price is now $165 but their website shows them being out of stock
https://t-cycle.com/products/assist-arms-catrike