On the excellent MoneySaversExpert site – probably the UK’s best site for impartial financial advice with a very active forum – I just discovered the Travel Money Maximiser page.
This lets you select the currency you want to buy (e.g. Thai Baht) and then gives you today’s rate for the major currency exchange bureaus, letting you see which one has the best rate instantly. If you’re travelling from the UK to Thailand, this is an invaluable resource to get the most Baht for your Pound.
You can also see daily exchange rates for all currencies versus the Thai Baht on Travelhappy’s Thai Baht Currency Converter.