Custom Enclosures!

1. Design Your Own

If you are willing / able to learn some 3D design skills, that might be the most flexible 
option. With a design and a 3D printer, you can design pretty much any kind of enclosure
you can imagine and modify it as your project evolves.

Here's a good tutorial about designing enclosures from Ben Heck: 

https://www.youtube.com/watch?v=03Ju_LJlU3U

Note that The 3D software he is using (123D) is discontinued, but the measurements and 
requirements gathering steps are still useful, and the process is much the same with 
other design software. The simplest to get started with is TinkerCad - 
https://www.tinkercad.com/.

If you don't have a 3D printer, you might be able to find a local hackerspace that has one
you can use either for free or for a small fee. http://hackerspaces.org/

If you don't want bother with the 3D printing yourself, there might be local 3D printing
services. Otherwise, you can use an internet based one like:

Shapeways : https://www.shapeways.com/
Ponoko : https://www.ponoko.com/
i-Materialise : https://i.materialise.com/

Expect to pay about $20-$30 for a Raspberry Pi sized enclosure, depending on the printable volume.

If you don't want to do the design, you could probably find someone on Fiverr to do it for you if
you can provide measurements.

2. Hack one together by hand

Here's another video from Ben Heck where he assembles a case using razor blades and sheet plastic:

https://www.youtube.com/watch?v=grG3EVPg1vI

3. Find an existing one and modify

There are simple existing enclosures at electronics places like Radio Shack. There are pre-designed 
ones available at places like Ali-Express or ebay:

https://www.aliexpress.com/category/200215202/cases.html

There's also a video that popped up recently on EEV Blog I haven't really watched yet on this topic:

https://www.youtube.com/watch?v=--oKMQITODc