It all depends on so many variables it's almost impossible to cover all the possibilities.
The difference in motors, sizes and plane types makes for a daunting task sometimes , but resources such as http://www.ezonemag.com/
Also this site also hosts another website:http://www.wattflyer.com/forums/
Do you have a specific plane/motor combination in mind?
What kind of performance are you expecting to get from a certain power package?
magazines such as past issues of "Quiet Flyer" :http://www.semodeler.com/, has many articles concerning motor mounting.
Personally I spent a lot of time on the E Zone reading threads and magazine articles in order to acquire enough knowledge.
While most electric motors sold today includea mount of some type, it may be necessary to either discard that mount or modify it, sometime by just adding spacers in order to extend the mount/motor to a point where the prop shaft extends beyond a cowl. or it can be a simple as attaching the motor mount to the fire wall with hex head bolts and T nuts.
There are a number of motor specific mounts available:http://www.espritmodel.com/, as well as those which can be ordered to fit a certain motor or motor/gearbox combinations.
There are a lot of options available to those in the electric community.
Congratulations on becoming a member!
Hope this helps.
cheers jollyroger