Well what you can do is look through Newegg at the different cases they have and then go through partpicker to look for the cheapest price. A case is a personal choice and like you just posted you don't like the looks so I don't know what you like and you are the only one who knows that so look through the cases and pick on that grabs your attention. Don't forget flashy sometimes means more money.
Every MOBO has holes in it for screws to attach it to a computer case. An ATX board has 9 holes. A ATX case will have 9 spots for those screws to go into, some cases have the "standoffs" already installed (small pieces of plastic that the screws go into to prevent the MOBO from shorting on the case), and some have spots for the standoffs to go that you have to place before installing the MOBO. I believe a mATX board has 3 screw holes in a different configuration than an ATX board. The case has to have spots to place the standoffs or spots where the standoffs are already that corresponds to the configuration of the screws on a mATX board. Many cases have spots for both and you just use whichever one matches the size of the board you bought (ATX or mATX).