Have you asked them if they can recommend a substitute?
I have one, but it's in Oregon City, which is a good twenty miles south of here. A long drive.
I'm making plans to make sure I get to the Portland Nursery Apple Festival this year. They always put out scads of various varieties of apples for their 'tasting'. I can hopefully make some worthwhile contacts whilst picking up a smattering of pie apples for this year's taste comparison.
In the normal course of events, the apples I have marketed to me by most of the fresh fruit vendors tend to be the sweet, for eating fresh in hand type of apples. Not pie apples. If you bite in to a good pie apple like you would a fresh crisp apple, it should make your face pucker up tight after the first bite. So, what is generally available to me for making pies are Granny Smiths.
The problem I find with Granny Smiths is that although they are nice and tart (usually), they also tend to be dense and firm. So much so that they don't readily bake down in a pie. To answer this, I buy a different variety (I prefer Braeburns, as they can be tart) to mix 50-50 with Granny Smiths so that I get a better final product.
Now....When I went to the Apple Tasting last year, I was quite late (like days after the event was over), but the huge pallet bins were still arrayed around the nursery lot and covered by tenting. I snaffled a bunch of apples (they did not have the variety I sought, so I experimented) of which the variety Idared was, by far, the best for a pie apple. Top rate!
In your part of the world, Moose, I think you should seek out and try some Bramley apples. Those are what the Apple Pie in Ambleside was touting as their source apples for their divine apple pie.
Granny Smiths are nice in hand apples, not pie apples. You need Goudreinet for a good apple pie.
In England, they recommend the Bramley.
Here in the US, I recommend the Gravenstein.
When I went to the Apple Tasting last year, the Gravensteins was the variety which had already sold out all stocks. This is why I sampled three other varieties, determining in the process that the theretofore untried Idared variety was just as good. I had never seen it in mainstream fruit or grocery outlets, despite it having been developed and introduced in the state which borders mine to the east, Idaho. The only place I had ever seen masses of this particular apple was right there at the Apple Tasting. Fast forward seven months and there I was in the Great Market in Budapest, wandering the main floor admiring the fresh foods when I noticed the sign on one piles of apples at one of the vendors....'Idared'. Then, once I'd noticed that, I noticed that practically every fruit vendor in the market carried the Idared variety apple, amongst others. Idaho apples are more readily available in Hungary than they are in Idaho.