I'll google Little Crafty Bugs. Shipping is from the US is probably going to cost more than the actual foam (If they are in the US) but I have really missed having the 3mm. Could you post again when you get it, to say if it is the right stuff?
Found Little Crafty Bugs, in the UK. But all I can see there is craft foam, which is firm, can be bent but is not soft and stretchy. Hope you do get the soft urethane foam.