AW's Li-Ion_sales_thread for most popular sizes of Li-Ions, including 17500.
AW's Protected_Li-Ion "C"_cell is 26535, ie 3.5 mm longer than regular "C" size, but might fit? I think that may be the nearest to 26500 you will find.
He does unprotected 17500s (further down in the same post as the protected), and I dare say you might be able to talk him into pulling the circuits off a couple of those "C" cells for you too, if you didn't want do do it yourself... Then the "C" would probably be a true 26500.