There are many types of cleanrooms available to meet your specifications. In choosing a cleanroom keep in mind the process inside of the cleanroom, materials used, chemicals used to cleanroom, and what your plans are for the cleanroom in the future.
Servicor Truly Modular Cleanrooms have been a growing choice for many as they can be reconfigured, easily upgradable, expandable, they are extremely durable, offer a wide range of materials for construction, and it can be written off as capital equipment instead of building improvements since it is removable.
For those in California it is also important to consider the need to meet and exceed Seismic Zone 4 Structural Calcs as well as IBC and UBC building codes. Many companies are not technically legal in California if they are unable to meet those standards. If an inspector sees this structure, they can shut down the entire operation if the correct paperwork is unavailable.
http://www.servicor.com has a ton of useful information and example of the different types of cleanrooms available.