A coaster brake is different than a fixed gear. A fixed gear has the rear cog attatched directly to the wheel, so however the wheel is spinning(speed and direction) is related directly to the speed of the pedals moving. So when the bike is moving forward so are the pedals. You cannot coast.
A coaster brake is an internal mechanism that causes the brakes. It has the normal freewheel mechanism, so that you can coast. But when you pedal backwords it causes the internal parts cause the hub to stop spinning which means the whole rear wheel stops.
What you are looking for is called just a single speed. Usually they are similar to road bikes, but with one gear and normal brakes. You have lots of options in all sorts of price ranges and qualities. Its a really open market depending on what you want, and how much you want to pay.