Do you want to buy Team Fortress 2 items (keys, hats, HATS!) outside the game client, throught the web browser ?
Well, the online store, at TF2 website, is currently closed. (Since 2014, or so.)
http://store.teamfortress.com
Currently, there are no way to browse for items through Steam Store.
http://store.steampowered.com .
But, you can buy using specific links to the items, which will add the items to your cart on Steam Store.
(This is a official and safe way to buy, as stated here: http://www.teamfortress.com/buynow_faq/)
For instance, the link to buy a key is: http://store.steampowered.com/buyitem/440/5021 .
(Note: 440 is TF2's game id, and 5021 is the key's item id.)
One way to browse for items, is to look for them in the official TF2 Wiki. There is a button/link under the item image, on the top right of the page.
Here is the key page: https://wiki.teamfortress.com/wiki/Mann_Co._Supply_Crate_Key
And, of course, you can also buy from other players at the (also official) Steam Community Market: http://steamcommunity.com/market/ .
