More than likley eBay.

http://cgi.ebay.co.uk/Ford-Fiesta-mk...item20b84156ed

There's a few on there.
Or you could go to pumaspeed if you got plenty of cash and order the american 2.3 unit with vvt.

To be fair though if you're looking at modifying you have a fiesta flame that you could get £1500 - £2500 depending on miles etc. Probably £100 KnN, £400 exhaust fitted, £550 cams fitted and remap around £300

That's 3.8k before you tell insurance that you have modded it
For that money you can get a Zetec-s or 4 grand for an ST Which are both quicker, better handling, better at braking and better equipped than your car would be after you spend the money. Plus, you then have an ST not just another fiesta