Someone in the armory thread posted a Punisher with a huge ammo can. He claimed that belt segments can be ordered by phone from some company. IDK if that is the distributor (Airzone), or the manufacturer (PTT?), but yes, it is possible.
Timebomb's thread discussing this: http://forums.humansvszombies.org/index.php/topic,2686.0.html
It was indeed Prime Time Toys.
This topic hasn't been discussed in over a YEAR, let alone I don't think the OP has been here since... Was it buzzbee or airzone that had some hotline or spreadsheet of the'r products so you could order individually?
Buzzbee form: http://www.buzzbeetoys.com/USglobal/AirWarriors/WebsiteOrderForm.pdf
I haven't used it, just googled it up.