Simply Be's advertised items sell out very fast so that may be why they are not available!
Please ensure you are entering the item number correctly.
An item number is 7 digits long (e.g. AB123CD). Simply type the number into the search box and click the search button. A page showing all the details of your item should be displayed. If you are told your item number is incorrect, check that you have typed in all 7 digits correctly, and that you have not substituted the letter I for the number 1, or the letter O for the number 0. If you still cannot find the item, you could try a keyword search instead.