LEGO-compatible suction cups? I'm looking for suction cups that are compatible with the LEGO pneumatic system.  I've seen people who have built things with them (on YouTube videos, etc), but haven't found anything on eBay or BrickLink.  Short of drilling a hole in a normal suction cup and popping a pneumatic tube into it, is there anything out there I could buy?
From the post about Vertigo (the precursor to Rory's answer), TrilogyGlenIvy (the creator) stated:


  Suction cup hangers were easily located in stores. Drilling a small hole and inserting a short section of copper tubing (1/8 “ or ~3 mm diameter) along with some vinyl glue produced the feet that I used.


Basically they are usually found in DIY/Home stores as metal or plastic hooks wrapped around a transparent suction cup so you can easily affix them to tiles or windows.

There is also a picture showing three variants that were tried, with the preferred option being those in the bottom left I think:



Depending on the weight of the model you've got you may also need to invest in some additional custom air bellows:


  An internet search finally pointed to a “Climb@tron Vertibug” toy that contained two air bellows (~23 mm in diameter). I bought one of the toys, removed the air bellows, and designed a “worm gear slider” system using NXT parts to stretch and compress them. The vacuum system was then installed in Vertigo and has worked reasonably well.