I did a search with google using "ammunition 2 1/2 12 gauge". I found that Cabela's carries 2 1/2 inch shotgun shells by Sellier and Bellot. Here is the link

http://www.cabelas.com/cabelas/en/templates/product/standard-item.jhtml?id=0012671210852a&navCount=0&cmCat=srchdx&cm_ven=srchdx&cm_ite=srchdx

I also found this post about 2 1/2 inch shells

http://www.findarticles.com/p/articles/mi_m0BQY/is_6_48/ai_85472055

that stated

"And the 2 inch and 2 1/2 inch 12 gauge shells are still with us.

Gamebore offers a complete selection of field loads. Bismuth markets a 2 1/2 inch waterfowl loading. Empty Fiocchi hulls are available in 2 inch and 2 1/2 inch sizes. And Ballistic Products even offers a complete reloading manual entitled Care and Feeding of Fine Doubles, with scores of recipes for the 2, 2 1/4 and 2 1/2 inch cartridges."

I did not find a source for 2 1/2 inch dies.

I hope that this helps.

Ron L