I've found the best resources to be museums. The National Maritime Museum has quite a few uniforms in it's collection and you can find a lot of them on their website. There were a few other websites I found some years ago, but I have lost them. Here are a few links to some info I think is pretty good.
National Maritime Museum
http://www.nmm.ac.uk/
You can view many of their exhibits online and search their archives
http://en.wikipedia.org/wiki/Royal_Navy_ranks,_rates,_and_uniforms_of_the_18th_and_19th_centuries
This is a wikipedia page so take it as you may, however this is an old established article I have found to be fairly accurate and quite informative
http://www.st-george-squadron.com/sgs/wiki/index.php?title=1748_Pattern
This is some research done by a society mate years ago and compiled on our society website. It has quite a few pictures, many of which came from sources like the National Maritime Museum