Try www.dvla-som.co.uk they were the best/cheapest for me. They start at �250 including the �80 transfer fee. Other sites/companies charge �249 onwards + the �80 transfer fee.
I got mine from national numbers. http://www.nationalnumbers.co.uk
You can also look on ebay.. I believe the guy is called chinesemervin .. I think he runs initial registrations (based in Ireland I think).
You should be able to get a dateless Irish plate for around �150. The dvla fee was �80 when I bought mine 3 yrs ago.