Filmmaking accessories like 3-axis gimbals, camera cages, and even sliders are helpful in getting a shot and introducing movement, but they can get rather pricey, especially if you're only going to use them once or twice. So why not 3D print what you need? Here's a list of some accessories you can not only print, but customize to your liking.
The first thing I needed for my Blackmagic Pocket Cinema Camera was a good camera cage, and I found one on Thingiverse. With a basic exoskeleton housing, plus rod mounts and a follow focus, this cage can be tightened down to give my rig a nice snuggly fit, and the cage will break before my camera does. Add a printed LCD hood and it looks very professional.
The next is a handheld shoulder rig, and on Thingiverse, there are tons different designs you can download for free, 3D print, and assemble. From complete shoulder rigs to individual parts like 15mm rod connectors and shoulder pad housings, there's a cornucopia with which to print a set design or create your own. My favorite is probably the self-proclaimed Ultimate DSLR Shoulder Rig with simple follow focus. The model comes with all the parts you need including connectors, mount plates, focus rings, and even viewfinder shield. All you need to add are the 15mm rods, and if you're on the budget, I recommend that old Swiffer you have in the broom closet. The size is pretty darn close.
How about a Glidecam? Can you really 3D print one that will operate smoothly? Well, the Kontinuum Glide 3D Printable glidecam makes the case quite convincingly. The creator of this model designed it for his Canon 60D and it comes with a series of felt and precision bearings to dampen out vibration as you move. The 3D model set comes with fully adjustable carriage, 3 axis gimbal, 2 piece handle assembly, and weight plate. There's also an optional central hub that can make the Glidecam operate even smoother.
Follow Focus would seem to be the natural 3D printed option, and again, here there are numerous designs ranging from a simple rocker arm extension to move your focus, to a lens gear system that can attach to that shoulder rig we were talking about earlier. There are even geared knobs to make fine tune adjustments, and even an array to house a servo motor with which to wirelessly focus your lens by remote control.
Camera sliders. Whether you use PVC pipe or metal rods, every 3D model site mentioned above has a few slider designs to try. The simple camera slider over at My Mini Factory, for instance, has a pair of 3D printable end caps and feet, with which to place 16mm metal rods, and then a mounting plate for your camera. The creator of this one also cleverly incorporated a motor from his son's lego set to provide motion by remote control.
When it comes to GoPro mounts, the sky is really the limit. From printable cages to even multi camera VR rigs, there's a printable mount for any need with which to put your favorite action camera right where you wan it, as well as knobs to screw it in to keep them secure.
I could go on, but it comes down to taking a quick search for the model you're interested in for the right application. And not just at Thingiverse either, there are other sites, as well, including My Mini Factory, Grabcad, and Autodesk, or you can take the shotgun approach and use 3D Model aggregation sites like Yobi3D, Yeggi and STL finder. Chances are, if you can describe it, you can find it and download it without having to pay a lot. But there are pay sites too that have incredibly complex and high quality 3D models such as CGTrader, Shapeways and Turbosquid. Some will even print them for you, for a price.
And if you can't find what you're looking for, you can always visit Tinkercad.com and design it. They have a basic set of online tools to get your feet wet in 3D model design, or to take an existing model and adjust it to fit your needs. There's also downloading Fusion360, which offers a free trial to create your own design.