The reprint history of TV21 strips is a long, hazardous journey that is continuing to this day. First off, you can’t rightly buy reprints of whole TV21 comics due to the various strips owned by different companies.
You can however buy strips series by series. Ravette got the ball rolling somewhat in the early ’90’s with a handful of paperback compilations of Stingray, Thunderbirds, and Captain Scarlet strips. Second-hand copies of these can be easily tracked down via eBay and Amazon.
Century 21: Classic Comic Strips from the Worlds of Gerry Anderson and Gerry Anderson: The Vintage Comic Collection all made valiant efforts in being definitive TV21 collections, but their luck sadly ran out. Both those titles are no longer in print, but Amazon and eBay can always help you out.
Egmont are the current forerunners in reprinting TV21 strips. You can currently buy The Gerry Anderson Comic Collection and Thunderbirds: The Comic Collection from them, as well as a handful of paperback titles, all five of which are collected into the Thunderbirds: The Comic Collection hardback. There’s also a second volume of Thunderbirds strips out later this year in October, so stay on this channel!