Portability... You will likely be taking this machine with you at times and there are light-weight machines that work really well. The easier it is to take with you outside the home the more likely your child will be compliant with treatment. Depending on the age of the child, it may be helpful to involve the child in the selection of the machine.
Health care equipmen. Stores that sell health care equipment usually also carry nebulizers. Also home health care companies will often bring the nebulizer to your home and teach you how to use it, as well as supply the medications. A prescription is required in order for your insurance to pay for it.
Varies. Some of the chain pharmacies will have these in stock. The most common source is a medical equipment supply house. It requires a prescription in either place. When you have the prescription written out, be sure they specify the tubing and medicine delivery device. The medications used would require a separate prescription.