You're welcome.
So the SS-5Form no help with a location.
Do you have their Canadian naturalization? Any luck locating the arrival manifest to Canada? What about the church your father and four of his siblings were baptised in? I would think that the church baptismal record would provide you with a parents place of birth.

http://www.kdejsme.cz/prijmeni/Posp%C3%AD%C5%A1il/hustota/
look in the left column at the names as how popular/common they are. http://www.kdejsme.cz/ click on the name.

I'm sure the Praha of old had more than one church just like now. The church records are indexed by a location and then religion.
http://actapublica.eu/