Found some cheap M42 extension tubes on ebay.
For about $12 dollars you get six (7,14,28 + 9,16,30mm) which
gives the following extensions.
7,9,14,16,21,23,25,28,30,35,37,39,4 2,44,46,58mm (missed a few!!)
7-14-28 $5.39
http://www.ebay.com.au/itm/Macro-Ext...23e3b1b&_uhb=1
9-16-30 $6
http://www.ebay.com.au/itm/Macro-Ext...d46d2e1&_uhb=1
Cheers
Stephen