Just what i would have said as i am in the trade. Size you want will be from an offcut of 4mm float.
The fitting will cost as it involves whats called a "Hack out" which is to Hack out all the old Putty and "wet glaze" the new pain in.
Just about all Decent Double Glazing Co's have a Glazing section if it has a shop front just dont go to any of the big Companys, Keep it Local and the price will sensible and should be able to quote you if you give them the size and let them know its a Hack out and wet glaze in.
Measure from the bead to bead and minus 5 mm.
The Glass thickness will either be 3 or 4mm depending on its age