I got mine here:

Here is a link that might be useful: Seed tray source

I use the White Perma-Nest trays. These are virtually indestructible and will last a lifetime. No more leaky trays! Additionally, the white trays will reflect light back to your plants.

If you buy 6 or more, you get a price break.

Here is a link that might be useful: White Perma-Nest Trays

Wordwiz,
I use Novosel as well, but they only have 1020 flats. Those are not the right size for 4" pots. Only 10 would fit and leave 2" of wasted space on one side.

Rokal,
The white ones wouldn't work for 4" pots for the same reason. The green ones however may work. I sent them an email. Thanks for the lead!

Rick