I don't believe you can actually buy it as a product per se. It actually has two parts - the straight part and the trigger part. if the straight part has hardened foam in it, a piece of a 1/4" OD vinyl aquarium air hose or a 1/4" OD poly irrigation tubing fits right into the trigger part.  The trigger part can actually be cleaned with acetone, too. If you're planning to reuse the straw, you can blow the foam residue out when you're done while it's still flowing.
That said however, I have to add that Great Stuff cans are made to be a one-time use product and they do have a very short shelf life (12-18 months). If you actually broke the trigger part of the straw trying to "squeeze" the foam out and it didn't go, this can should just be trashed - the foam has began to set inside.