We get a lot of static electricity in our office and an electrician visited and said we should put rubber mats underneath our computer towers (as they rest on the floor). I have been looking round but apparently we don't need a grounding mat as it isn't relevant but I wondered if you could advise where I could buy the right rubber mats from?

Is your office carpeted or tiled floor? Google rubber mats for computers and add your zip code.

Most PC Frames are designed to disparate static electricity through the frame grounding system. If not, motherboards and components would be replaced would be replaced on a horrific pace.

Just about any rubber mat would insulate the PC from the floor, but do noting for a static charge carried on a person (eg. Touch a door knob and get a shock)

Not to get off your original question, but adding humidity to the office would be a good start... not to mention the other benefits with properly conditioned air... for both the office inhabitants and the equipment longevity.