Excel 2007 : stop duplicate items in a range

I have a stock list of printer toners. They are labelled Axxx. eg

1 A B C D
2 HP 760a A12 A34 A136
3 HP 11a A223 A333 A410

I want, from the range B2:D3

the current highest Axxx to be put a cell so I don't have to read all the range to get the next one

or to be prompted for the next Axxx when I type it into a cell

or be stopped from typing in an Axxx below the highest item.

I've tried COUNTIF etc - works fine on numbers but not text and numbers. COUNTIF only seems to work on a column, not a range.

I don't want to use a macro.

Thanks.

Paul McLaughlin