Used wheels from tri-bikes will be a good bet. Or buy a rim and build. Theres the standard open pro readily available http://www.chainreactioncycles.com/SearchResults.aspx?Search=650c+rim&_$ja=kw:650c+rim%7Ccgn:650c+rim+-+R%7Ccgid:4125929727%7Ctsid:40917%7Ccn:Chain+Reaction-SE-G-Exa-NonSwe-DT-SE%7Ccid:91912767%7Clid:2812558821%7Cmt:Exact%7Cnw:search%7Ccrid:12069214887&gclid=CKv0qtiBxbMCFS56cAodbRQAJw
Heres a bit more rim depth: http://www.universalcycles.com/shopping/product_details.php?id=31903&category=856
@badboybjorn started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
Used wheels from tri-bikes will be a good bet. Or buy a rim and build. Theres the standard open pro readily available http://www.chainreactioncycles.com/SearchResults.aspx?Search=650c+rim&_$ja=kw:650c+rim%7Ccgn:650c+rim+-+R%7Ccgid:4125929727%7Ctsid:40917%7Ccn:Chain+Reaction-SE-G-Exa-NonSwe-DT-SE%7Ccid:91912767%7Clid:2812558821%7Cmt:Exact%7Cnw:search%7Ccrid:12069214887&gclid=CKv0qtiBxbMCFS56cAodbRQAJw
Heres a bit more rim depth:
http://www.universalcycles.com/shopping/product_details.php?id=31903&category=856