Apogee Components, Aerotech and Public Missiles are just a few of the companies that offer large model rocket kits for sale. Large model rockets that fall into the high-power rocketry class must be launched by a certified individual.
Model rocket kits vary drastically in their design and required skill level, though each rocket, large or small, contains the same basic parts. Many smaller kits are very simple and can be launched by anyone with a back yard. As the kits become larger and more powerful, they require a larger space in which to fly. This is because as the rockets go higher, the prevailing winds get stronger and can carry the rocket miles from the launch site.
When a model rocket reaches the threshold of 1500 grams of total weight and contains a more than 62.5 grams of propellant with a rating over 160 Newton-seconds of total impulse, the model is considered high-power. At this threshold the rocket becomes governed by the National Fire Protection Association. To launch a high-power rocket, an individual must be certified by that country's sanctioning body. The specific weight and power thresholds can vary between countries. Be sure to check with the sanctioning body before launching a large model rocket to ensure that the launch complies with the proper guidelines.