Have you developed a requirements list yet? This could help you with your design. Depending on the motion requirement, you may find that a suction cup is not the best choice (it might be otherwise).
In addition to the suction cup, have you considered using some form of non-transferring contact adhesive (like Scotch tape/masking tape)? Will the robot need to endure any specific motion?
McMaster is a decent place to start looking at materials:
http://www.mcmaster.com/#standard-vacuum-cups/=ldybw8
Suction cup tape:
https://www.inventables.com/technolo...ction-cup-tape