I've having such a hard time getting these and its frustrating because my chams love them. I used to get them from http://www.silkwormstore.co.uk but it doesn't always have stock, he recently sold me his last 20 of medium and gone already
I can't help with finding you any at this moment but when you get you next batch don't feed them all off. Keep some and let them breed. You'll always have a supply and be saving some money.