if it were an out of print deck that were extremely hard to find and no one here could spare a replacement card from an extra deck (understandable), I would do what Cassandra022 suggested and create a replacement that consisted of the title of the missing card.
I would make it either using the title card or out of paper that I would laminate. It wouldn't matter if it didn't blend in to the other cards either visually by card backs or in tactile fashion; I would circumvent that problem by counting out the cards for readings so I can't subconsciously avoid (or choose) the "special" card. I already do that with two cards that are conspicuous for different reasons in my favorite deck, which also happens to be self-published and expensive-to-replace.
I wouldn't be happy about having to do all these things, but I those are the breaks when you're working with something that is either not out in the market or is far too costly on that market to be able to replace the whole deck. I would still use the deck if it were one that I liked very much, and be grateful I had found a way to use it.