You can use any kind of LEGO® bricks but we always advise people to use bricks from the LEGO® SERIOUS PLAY® product range.
For shorter workshops it is very useful for participants to each have a Window Exploration Kit to start up with. They are sold in boxes of 100 bags for app. 485 USD at LEGO.com
The starter kit is a good choice for longer workshops. It features LEGO® and LEGO DUPLO® bricks and elements including wheels, tires, windows, trees, minifigure parts, sticks, globes and small base plates. These boxes are sold individually for app. 37 USD at LEGO.com
The landscape kit or similar amount of bricks is neccesary for longer workshops (+2 hours). The LEGO® website states that these bricks are suitable for workshops with 10-12 people for 2 days. Our experience tells us that the bricks are suitable for 20 people for 2 days. These bricks also last a long time and can be used over a series of workshops. This set can be purchased for app. 790 USD at LEGO.com
If you live in a country that is not supported by the LEGO.com website, you can read more about ordering the bricks here
Furthermore you can buy pre-used LEGO® at ebay. It can also be a good idea to purchase extra mini figures or LEGO DUPLO® animals.
For workshop where participants create shared models, you also need a big baseplate.This models is usually kept by the participants. You can purchase these in toyshops or at LEGO.com