As far as I know they aren't available separately. I'd suggest finding someone with a worn cassette where they haven't used the 17 much - but it is one of the likely favourite sprockets, and therefore one of the most worn.
The 17 is likely to be only a single sprocket on the less pricey versions - Record and Chourus tend to have pairs of sprockets higher up the numbers.
I've always just put on a complete new cassette, on the basis that if one sprocket jumps the rest of the middling placed ones won't be that far behind