Picture postcards didn't get produced in the US until the 1870s-1880s and then in fairly limited designs; the flood came with the collecting peak in the early 1900s. So realize that one from Poole's time or just after his death is likely impossible. A later card could be possible, even a real photo type, but it might turn out to be the same image as on your repro. Don't limit yourself to just looking at US postcards.
For a start I suggest you set a permanent search for William Poole on
ebay, if you haven't already done so. There are also aggregator sites for traditional-type general auctions to search.
Based on notoriety, I would guess a cabinet card of him would sell for $100 or more. The price will depend on your competition in the overall marketplace.
For such a specific subject, I think it will require patience to turn up what you want. In the meantime though, you might end up finding and buying other Poole memorabilia. Good luck!