That has been my problem in the past. I had trays that were held together by duct tape. However, my solution was local. Check the garden centers starting in January or Feb, that's when they start bring out the plant trays. I was able to get trays for my indoor and wintersowing (2 different type of trays) at price cheaper then the big box stores or walmart.

I found these today. Anyone use them?

Here is a link that might be useful: quad thick trays

I had that problem too. Finally I just ordered triple amount of flats from on-line supplier and tripled them up. The cost is more at first, but I am so happy I finally did that. 3 trays stacked together makes them sturdy enough that I can move them back and forth without worry. The sun does make them brittle, and after awhile cracks right in the middle started to form, but so far they are still holding up great.

I'm not nearly coordinated enough to maneuver two full trays at once without accidently dropping one!