Research for a digital marketing course

Hi there.
I am currently taking a digital marketing course and the final project is about digital marketing agencies. I'm looking for some data if possible on the number of agencies in the UK and/or USA by year (to see a trend of an increasing number of agencies) if there is one..
Also if anyone has any information about advertising spend by agencies per year then that would be great as well..
Any ideas/information would really be appreciated.
Many thanks
Oliver.

See original post by Oliver King