Your input appears to be ready sorted so I think you just need something
like something like (untested)

<xsl:value-of 
 select="/beers/beer[contains(concat($searchLetter,substring-after($alphabet,$searchLetter)),substring(.,1,1))][1]"/>

David

_____________________________________________________________________
This message has been checked for all known viruses by Star Internet
delivered through the MessageLabs Virus Scanning Service. For further
information visit http://www.star.net.uk/stats.asp or alternatively call
Star Internet for details on the Virus Scanning Service.

 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list