Missing the letter B on the keyboard, how do i obtain a new one

Missing keys are difficult to obtain, because the hinges under the keys vary, even in the same model computer. You may be lucky to get a discarded keyboard that matches exactly with yours, but the best bet is to replace the keyboard with a new one.
Keyboard replacement are available from this WEB site - click on this link :-
http://www.yallstore.com/index.php?main_page=advanced_search_result&search_in_description=1&zenid=1lrjst3ncshlb4c7frn9bev3c3&keyword=keyboards

On Ebay the key will cost about 5 buck, or the entire keyboard could be replaced for around 20 bucks. Here is the service manual just in case you want tochange the entire board......Check out page 46....

http://tim.id.au/laptops/acer/travelmate%20505.pdf