There are folks who have a prop closet, or something of the like. Contained therein are usually any variety of things like costumes, guns, swords, skeletons, armor, etc. Such props can be quite useful when trying to paint something with a great deal of specificity.
So what do I have? Not a whole lot, actually. Sure, I've got a sword and a couple of flint-lock pistol replicas. I have an entire replica Union uniform from the American Civil War from a previous life when I used to reenact. I've got various hats that I've collected, an old M1 army helmet, and a number of other costume bits and bobs, but nothing of the ilk that would make other illustrators jealous. Still, I employ the various props I've collected as effectively as I can to add a degree or two of authenticity to things I've portrayed.
What about the rest? Well, I fill in the blanks as best I can.
The first thing I do if possible is build a model or replica of a given prop if I think it necessary. These can be cut and glued from cardboard, wire, or even sculpted from clay. I've built replica shields, swords, and armor using all kinds of stuff that regularly ends up in the recycling pile.
Other times, I'll try and dig up something that does a fair job of replicating aspects of the object I'm trying to depict. For example, if my character has a metal staff that's highly reflective, I might use a vacuum cleaner's chrome-plated pipe to stand in for a metal staff, so I get a good idea of what might reflect where.
Simple enough, right?
The key is to find things that will give you the information you need. If it's about how light falls on a given shape, then maybe you need to build something and fill in any blanks with reference taken from books or the internet. If it's about how it reflects or appears under specific lighting conditions, then perhaps you'll need to hunt one down if it exists, and shoot pictures of it yourself. It's all about figuring out the most important things you need to nail and then finding a way to get the information you need to make that happen.
First off, one of the greatest resources known to illustrators nowadays is the internet. There are thousands of images of environments from which to draw. Finding images of cathedral interiors, stables, or the cold of space are pretty easy to stumble across — especially since things like Google Images and Flickr came along. Plus, finding specific places is pretty easy, as well. Still, you shouldn't rule out other resources like your local library, which often times can be even more useful as you can find entire books about a specific place you're trying to depict.
Now, for those of you who have access to the type of environs you're trying to depict, there's really little you need to worry about. Otherwise, it's all about piecing together what you can and creatively presenting things in a way that people will buy into. Vague, I know. But, it's really all I've got.
Look, thing is, if you're tech savvy, then maybe you're the kind of person who can actually create a digital 3-D model that you can turn and rotate and view from any angle. Maybe that's how you figure stuff out. Maybe you do something like James Gurney might and build a model of your environs from scratch and then inform it with all kinds of reference to make it all the more believable. Maybe you're like a lot of folks and you just figure out a specific image you want to base things off of then riff on it and bend it to your needs.
I, myself, am not a big environment guy...apparently. Looking over the work I have on my site and have sitting around in my flat files, it's clear that I'm not into the elaborate mosaics, or wall paper. I don't do a whole lot of really heavily detailed landscapes or interiors. Part of that has to do with my sensibilities as an artist or illustrator. Part of that has to do with the needs of the assignments I work on. Nevertheless, I have thousands and thousands of photographs that I've taken and collected over the years of environments. I go on vacation, I take photos. While other folks are looking to take photos of each other in front of landmarks, I'm muttering under my breath, waiting for the tourists to move on so I can get a clear shot with fewer people in the way. My wife and I have gone on entire vacations without once photographing one another or being photographed together. (Romantic, I know, but it's something we've always done). Without some of these photos, there are pieces that would have fallen apart and would not be something I show folks today.
Like with props, and all other bits of reference, dealing with environments is all about understanding what you need and then finding a way to cobble those things together. Again, not everyone uses reference the same way, and not every piece requires the same amount of or the same dependency on reference. As such, you'll develop your own ways and methods to get what you need not only in general, but on a case by case basis.
Then, once you have it, it's all about figuring out what to do with it.