What is the best resource for Card Trays?
Without parameters on shape, size, and cost, it's difficult to give a good answer.  However, if it were my game and I needed 4-5 6-slot horizontal card slots, I would consider making them out of folded cardboard.  With a little trial and error it should be simple to shape what you need from 7x3 (21) horizontal folds, and 3 vertical folds with 2-4 tabs to keep the shape.

If you want plastic, you likely want to purchase your own vacuum former.  Hobby versions can be had for as little as $120 (plus materials).  They work by laying a piece of plastic over a mold you create to form any plastic shape you want.

Random site selling a former