In the UK you have to get ISBNs from Nielson, as stated by buddhuu above. They are the only accredited agency, and you have to buy a block of 10 ISBNs - they don't do them singly - which will cost £111.86. Here's a link to their ISBN FAQs:

Getting ISBN details from Nielsen

ISMNs are for music scores and the relevant agency here is the Music Publishers Association. Here's the link:
Music Publishers Association ISMN info

I think you can get individual ISMNs from them but don't know the cost. Song books apparently qualify as 'crossover' products and so can have either ISBN or ISMN .. .or both!