This is What You Will Need
Besides a suitable topic and large table, you will of course need LEGO. Here you can get an impression of the LEGO that you will need. The required LEGO is no other than what you might have stashed in the attic, or what you can buy at the toy store. You can use all LEGO, the most important thing is that you have enough variety and quantity. When collecting LEGO parts, the most important thing is to consider that the LEGO will act as a metaphor.
To illustrate: if you have a little fence in your hand, this could represent the deliniations of a garden, but the fence could also be a metaphor for something from which someone feels blocked, or just as a metaphor for safety. By having a variety of LEGO parts available, the participants can become inspired and find something which can serve as a metaphor.
There are ready-made kits available for LEGO Serious Play, for which the cost is quite high. You can also assemble the LEGO yourself, for this I have drawn a plan of action. It may be that you have some LEGO at your disposal yourself; you can always expand on this by buying LEGO-parts.