Here are some quick links to think tanks and organsiations both for and against. Most have news and reports on their website.
Yes to AV - formed from the original make my vote count campaign.
Take back Parliament - campaign group
Unlock Democracy - incorporates materials from Charter 88
Electoral Reform Society Long term campaign group. Website has numerous briefings and reports.
Compass - associated with the left recently published a pamphlet Democracy by Machines or Morals?
Operation Black Vote campaign group
Negative aspects of AV
Centre for Policy Studies Evaporating Case for Electoral Reform
GMB union - truth about AV
Other papers assessing the possible changes include.
British Academy report: 'Choosing an Electoral System':prepared by Professor Simon Hix, Professor Ron Johnston FBA and Professor Iain McLean FBA with research assistance from Angela Cummine
Constitution society briefing paper
Hansard society has some archived articles.