I use a mix of Sabol Trays and "Blu" Trays (which I find to be stronger than any other tray out there). If you have the bag, you may as well fill it up with trays. Sabol could be obtained from TheWarstore or any
FLGS. "Blu" Foam is something that one
FLGS in my area sells, and the only place on the internet I've found it was Miniatures Market. I've bought both, and it's the same stuff:
http://www.miniaturemarket.com/inc/sdetail/756466
If you already have somewhat of an investment in destroyed Sabol trays, consider putting them in my favorite carrying case:
It goes by many names, it's cheaper than any other case out there, it's collapsable, it's made of strong hard plastic, and is available in many places. It's strong enough that I can carry my models through the New York Metro just fine. You can usually find it at Office Depot, Office Max, Home Depot, Lowe's, or even Bed Bath and Beyond.
One thing you may want to do is keep things consistent. If you have/use Sabol-sized trays, keep using Sabol-sized trays for your entire army. You can use Sabol Trays, BLU Foam, Outrider Foam, or even Battlefoam in Sabol Size. Sabol also sells an "extra large" foam base that's twice as wide for storing things such as Baneblades or Land Raiders. People seem to love KR for their price, but their sizes are not compatable with Sabol; considering you already have a case, it may not be the best choice. Same with Battlefoam and
GW, but those are different sizes [i]and[/] they carry a large price tag.