Yes, there's rebuild kits available.  Here's Kragen's offering:

http://www.partsamerica.com/SelectParts.asp?SourceArea=SHOP&PartType=237&PTSet=A&Description=Clutch%2C+CV+%26+Drive+Shaft+Parts&SourcePage=PartTypes&SearchFor=Clutch+Slave+Cylinder+Kit
---------------------------------------------------------------
Tom Young                       '81 Vanagon
Lafayette, CA 94549         '82 Westfalia
---------------------------------------------------------------
----- Original Message ----- 
From: "mark thoma" <thoma@MINDSPRING.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Wednesday, April 23, 2003 7:40 PM
Subject: Any cheap fixes for clutch slave cylinder?

Hi all,

$50 to $100 for a replacement slave cylinder for my beloved van seems a bit steep when all it probably requires is a new rubber plunger on the inside.  Anyone aware of any rebuild kits available?

Mark Thoma