Does any one still make 26" Mountain bikes? I'd also like to know what would happen If I tried 26" wheels on a bike bought with 27.5 wheels. The 27.5 bike is just too big for me even with the smallest frame size.
There are a variety of bikes with the same wheelsize. A few companies, such as Surly, still make mountain bikes with 26" wheels. A lot make 27.5" wheels, and a ton make 29" wheel based bikes. I believe Jamis also made 26" wheel bikes until quite recently.
Provided the hub spacing is the same and you have disc brakes and an appropriate rotor, you can just slot in a 26" wheel into a 27.5" frame provided the spacing is the same (which it likely is). If you have rim brakes, they won't line up and you can't use the wheel. Note that the riding position and what not can change.
A lot of people have done 69er or 96er bikes in the past where they've taken a 26 or 29" frame and put in a wheel size of the other.
The size of a bike has very little to do with the wheel size. For example, if you look at the Surly Instigator with 26 inch wheels, the smallest size has an effective top tube of 55.5 cm, and a reach of 37.8 cm. Compare that to the Trek X-Calibre which on the smallest model uses 27.5 inch wheels but has an effective top tube of 52.5 mm and a reach of 35.2 cm. For road cycling just about all riders use 700c wheels and they pretty much accommodate all riders.
Putting smaller wheels on a bike isn't going to change the reach which is a very important part of how a bike fits. The standover height will be lower, but mountain bikes generally don't have a huge problem with standover height, and the seat can usually be dropped pretty low. And if you put smaller wheels on the bike, the bottom bracket and pedal clearance will be lower than designed.
What is your height? Which brands have you looked at? Big brands such as Trek or Specialized have a large range of sizes. Some models use 29 inch wheels for the larger sizes, and 27.5 inch for the smaller sizes.
There are plenty of mountain bikes with 26" wheels, and they're not hard to find. I have no idea why you'd even ask that.
If you have disk brakes, then yes, you can put on smaller wheels. The only problem with that: your cranks will be closer to the ground, so there's a greater chance of hitting your pedals when you're cornering or riding over a log or something. Depending on your bike and what you use it for, that may be something you can live with.