If your looking for somewhere where you can actually go in and buy it - i always get mine from Marks and Spencers. They always have a competitive rate - think it is about 1.84 at the moment!
OK for years now I have used http://www.travelex.com
They have the best rate and are commission free. You can pick the money up at the airport or have it sent. Its brilliant!
Changed some money today at a rate of $1.88 at local Ulster Bank here in NI.
they are part of the RBS i think so would imagine the rates should be similar