If you're looking for the places to purchase Tunnels and Trolls items online, the best places to go are the Flying Buffalo website, RPGNow (aka DriveThruRPG), and Lulu.
Flying Buffalo is a straightforward enough choice; they are the official publishers of T&T after all. Go to their website to find an array of new and old adventures and supplements. You can find their T&T page here.
If you enjoy your RPG accessories in PDF format, one of the best places to shop is RPGNow (also known as DriveThruRPG). There are currently 86 different T&T related items available on RPGNow including the complete 7.5 Edition box set, GM adventures, solo adventures, rules supplements, and magazines. Some of these items (like TrollsZine!) are even free. For a quick look at what is available go to the RPGNow T&T items here.
If you still prefer your gaming material to come in a nice paperback book, check out the print-on-demand publisher Lulu. There you can find a number of solo adventures, GM adventures, magazines, and even the French edition of the T&T rules. Check out Lulu's T&T list here. Of course when shopping with Lulu you should always wait for one of their many sales and save a lot of money, so be sure to register before you buy.
There are several reasons to shop at these sites. In most cases, you will be purchasing directly from the authors, especially when shopping at RPGNow and Lulu. You will also get the lowest prices when you do so. There's no reason to pay outrageous mark-ups from second-hand dealers; buy direct. Finally, by purchasing from the authors of these products you help to encourage the production of new T&T material which is something that we all want to see.