Any time you ask to speak to a supervisor, you should have that opportunity afforded you. She, the rep, did not have the authority to put you off for so long. My wife's uncle used this ploy when talking with a customer service department - If, after a while, his question looked like it was not going to be answered, he asked to speak with a supervisor. When he got the super and it still looked bleak, he asked to speak to that person's supervisor. Usually, the person on the phone was only too happy to kick the problem upstairs. He once got all the way to a Vice Presidential level at his local utility. Doesn't always work but, if you persist, politely but firmly interrupting the rep's spiel, it may get you there.
PA should wise up and ship a partial order when only one item is backordered. I think I saw some photos of their shop and it didn't look all that big, inventory-wise. It's one thing to use "just in time" inventorying, but you can't continually backorder stuff, especially for three months.
Want pellets? Try Tim at Mac-1. His "whole lotta lead" sale is now a full time thing. Might not be as cheap but haven't heard anything bad about them at all.