For travellers cheques and currency, you will struggle to find cheaper than Travelex, providing you book them online at least 48 hours before you travel. Booking with Travelex is also very convenient as you simply pick up what you want when you arrive at the airport - www.travelex.co.uk
Where are you traveling to? Check the exchange rate of that country and you can calculate from that. The bank might be the best. At least it always is for me.
I normally ring a few places and ask them directly how much will X euros cost me including any charge you may have? that way you can see exactly how much each will cost you - you need to be aware of places that advertise 'commission free' as they can sometimes offer a lower rate of exchange....