This is one of the more difficult things to do. How do you want it to look? Do you have a drawing, or a plan on how it will look? Does it show 'you' in the suit giving approximate dimensions, etc?
Digital Morphology - xrays and pics of animal skeletons - http://www.digimorph.org/
Timduru's page of pics of animals from zoos and related - http://animals.timduru.org
A site on animal pics for references - http://animalpicturesarchive.com/
Veterinary charts, may be good in some cases - http://www.tri-esssciences.com/veteraniancharts.htm
Animal pictures at Sunet's site - ftp://ftp.sunet.se/pub/pictures/animals/
Costume Network - magazine and gallery featuring unique and fantasy costumes - http://costumenetwork.com/
LiveJournal's Fursuiting For All - http://www.livejournal.com/community/fursuit/
Julie Zetterburg's costume pages from A to Z - http://members.aol.com/nebula5/
Courtesy of Tashta, site on making various suit parts - http://www.stripedsmiles.com/19CostMakingSt.html
How to get a head without hunting - Nice site! - http://www.animalhead.com/index.html
Though non-fursuit related, an interesting page on how this guy made some costumes. Some can be applied to suits - http://www.cockeyed.com/incredible/incredible.html
SuperJay's site with lots of links to other suiter's sites, great for visual references and other helps - http://superjay.timduru.org/
Durin's page on some interesting methods of making a skunk fursuit - http://durin.treeweasel.com/