I wouldn't make prints etc. of a commissioned painting.
Although the copyright on the painting normally remains with you, commissions are a grey area because the client has asked you to create something specific according to their direction / idea or photo so also has claim to the originality of the artwork in the same way as somebody that commissions a ghost writer to write them a story.
From another perspective - commissions normally have a lot of emotional value attached to them so would you be happy seeing the artists selling prints to Joe Soap and his friend of this artwork that means so much to you?
What I suggest you do if you really do want to sell prints of a commissioned artwork is to ask permission from the owner of the artwork. If they say it is okay, get it in writing. They may ask for a percentage of sales, which in my opinion would be warranted.
The better option is to create unique artworks that you never sell which can only be purchased as prints. That way you don't devalue your originals as they remain unique and the prints are exclusive.
