I don't know if they sell individual chips, but this place is worth a look:
http://www.ashlea.co.uk/scripts/availability.pl?partID=XC3S400-4PQG208CThe last time I enquired about Xilinx XC3S400-4PQG208C FPGAs I was asking for the bulk price, which for 20-100 chips was £13.40 GBP each. I didn't ask for the price for single chips.
Even if they do sell individual chips, it looks like you've got enough interested parties, i.e. people who have posted here, for at least 5 FPGAs. Something to think about.
If you would like to e-mail Ashlea Components their e-mail address is: sales@ashlea.co.uk .