There have been a great many Tintin neckties produced. For each 'range' that the Tintin Shop, in cooperation with Moulinsart produced, there were a selection of ties. They used to be brightly coloured designs, with lots of small images of the characters ; recently they have produced more formal designs such as the endpapers of the black and white books, a large headshot of Tintin in two-tone, and Tintin's signature repeated across the tie.
Before Moulinsart started producing ties, the company 'Studios Aventure' (I think) made them, including a rather superb reproduction of the cover of
The Blue Lotus in full colour.
Regarding availability, eBay is a good place to check -
eBay.fr often has some, if you search for "Tintin cravate".
Tintin.com's official shop has some of the recent designs for sale, priced at 45 EUR each.
Hope that's of some help !