There are normally three places model numbers can be found.
Firstly at the bottom of the front plate just under the door (you normally have to open the lower door to see\read it).
Secondly for integral ovens on the lid, its just a case of removing 4 screws and pulling it out and a label and barcode can normally be found on the top.
Lastly to the rear of the oven, normally located on the metal panel near where the electrical feed goes in.
If you still cant find the model number, there are universal control knobs you can buy, have a look here.