I have a few hornworms now also and besides using the mulberry chow that I already had, I put a few tomatoes in their container and they seem to be fine with eating that.
http://www.manducaproject.com/
That site is one that I used to find some information on the worms, so try looking there to...
You can order silkworm holders online. I have one and its just a little clear box with a blue top. Its very simple.
You can definitely hold 75 worms in there. I have had at least 100 in there at a time, with no issues.
I don't know the website that I ordered it off of at the top of my head...