What is a mbuna tank? New to this

Mbuna are a colorful and active group of cichlids from Lake Malawi. They live in and around rocks and graze on algae that grows on the rocks. What size tank are you looking for? Most will say a 4' tank is a minimum, though with the right species a 3' can work. You will want to pick out a few species and have groups of 5 of each species (1 male:4 females). A mbuna community is different than a regular community because mbuna can be very aggressive and territorial. Oddly enough they like to be in groups. Over stocking a tank will help to control the aggression, as will picking the right species. Beaware than many commonly available(and pretty) mbuna are psycho killers and can wipe out a whole tank if they feel like it.
To set up a successful mbuna tank it's a bit of a balancing act of finding the right species compination, overstocking, and providing lots of hiding places.

Also welcome to the world of mbuna!

Once you get the stocking right it is a rewarding and fun tank to sit back and watch. Many people ask me if the tank is saltwater since my fish are so colorful.

A 100 gallon is a great set up. I'm not sure how many species you could go with. I would think 5-6 species would be a safe mix. Demasoni are great fish but are hyper aggressive. Either get 1 or be prepared to get 12+. Having a larger quantity will keep them from killing each other. Since you have plenty of room in a tank that size it would be fine.
Some other colorful and more behaved species are pseudotropheus acei (purple with yellow fins), Labidochromis caeruleus (bright yellow with black fins, also known as yellow lab), and Iodotropheus sprengerae (known as Rusties, they are a rusty color with some purple). These three are some of the best behaved and commonly available mbuna out there.

Also be sure to look into the Cynotilapi afra species, a lot of variation in that group of mbuna and they see to be middle road as far as aggression goes. I myslef keep Cynotilapia sp. "White top haras". The males are white blue with black stripes and the females are a nice grey blue as well.
Another great beginner species is Pseudotropheus saulosi. Now you could not keep these with demasoni since the males look similar. They are better behaved than demasoni. The males are blue with black stripes and the females are bright yellow.

Calvus are from another lake all together and will not mix well with the aggressive mbuna.

It depends. I have gotten most of mine from breeders in my area, and a good lfs. If you want even more options then ordering online is a good place to get stock from. Though shipping can be pricey.

Be aware of any "Assorted African Cichlid" tanks. They can contain hybrids, if you buy from one of these tank make sure you know what you are doing. Sometimes there can be good finds.

here is alink that will tell you a little bit about them

http://www.aquahobby.com/e_african_cichlids.php