How about using a hex head bolt, turning or filing it down to size, & cutting a slot into the heat.
Another option might be to simply use the longest bolt which you can find, & epoxy
bond it in place, sans nut. If you've got 3"-4" bonded in place, the head of the bolt will tear off before the bolt will come out. Assuming, that is, that the bolt's bonded into something stronger than 6lb Foam.
If the substrata currently isn't of a high enough strength to bond a bolt into, can you replace some of it with something which is? I'm not clear on the why behind the need for such a long bolt. Particularly one so thin for it's length.
There's one catch with bonding it, & that would be getting it out later if you needed to. In which case, bond a shorter bolt in place. One that'll come out once it's heated up good.
Since McMaster-Carr's already been mentioned, how about Jamestown Distributors?
That, or just have one custom made, if having an exact duplicate is critical.