For a while I have referenced a site called www.posemaniacs.com where you will find tons of poses of computer sculpted models . The best part about this site is that you can rotate around the poses to see them from all sorts of angles.
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Another great option for models of people and other objects like cars, helicopters, building etc. can be found at www.3divia.com . This site is just amazing because once again you get to rotate around the poses and the objects. You have to do a quick download of there software but I've found it useful so far. I've only used the free models but I can just imagine how amazing the others must be judging from the free samples. How often do you really get the chance to rent a helicopter and take a photo of a house from above anyway? obviously you could spend hours looking for reference online but you won't get to choose the exact point of perspective you want unless you draw it all out, happen to know a drafting software or own a helicopter.
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There is also the whole area of creating your own physical models out of clay, paper mache, wood, found objects etc. This works pretty good for very specific characters you've designed for a project. For example you may design an alien with all kinds of strange limbs so the best option most likely is to build your own model. For one of my books I got to do a bunch of illustrations of Noah's ark so I decided to take a few hours to actually build a small replica out of wood, shown below.
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One final option I've used is called google sketchup which you can actually download for free and legally. This program allows you to build things from the ground up and then you can rotate around, under, above, zoom in and out etc. The trouble is that you will have to spend a bit of time figuring out the software but it's actually not too hard. I actually used this program to redesign my studio room. Now you could go all out and by the expensive programs but who really has the money for those?
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