Taken from the dvla web site:
Q: Are stick-on number plates legal?
A: No, number plates must meet the British Standard requirements and we are not aware of any self-adhesive number plates that are able to meet these requirements.
http://www.dvla.gov.uk/faq/faq_number_plates.htm
However I have a caterham with a stick on front plate, and never had any problems. Most of the classics at Goodwood yesterday had the same too.
Yes you can buy them, but have to shop around, as most places want to sell you plastic ones now. Trailer supplies or boat chandlers are favourite.
Try:
www.craigsplates.com, or
www.fancyplates.com
Cheers,
John.