"And, where do I get the plants/seeds? "
This really depends on where you live. In California yes you may cultivate plants for your personal use. This is an excerpt from SB 420:
11362.77. (a) A qualified patient or primary caregiver may
possess no more than eight ounces of dried marijuana per qualified patient. In addition, a qualified patient or primary caregiver may also maintain no more than six mature or 12 immature marijuana plants per qualified patient.
Here is the entire bill for your records: ftp://www.leginfo.ca.gov/pub/03-04/bill/sen/sb_0401-0450/sb_420_bill_20031012_chaptered.html
It is important that you review the laws stated within your particular state as personal growing is not allowed in every state. Each state should provide a government site that lists the particular rules and regulations for where you live. Another thing to note is that within California the laws will be changing in November, so at some point the above link and statements may change.
I would agree with Pamela that 6 mature plants OR 12 immature plants is a safe bet; however, I would encourage you to look up the laws governing your county. Some counties have banned any growth, while others are more flexible. For instance, patients are allowed up to 24 plants within a 25 square foot canopy in San Francisco county, and dispensary grows are allowed up to 99 plants within a 100 square foot canopy.
The site CANORML.org tends to stay up to date on current federal, state, and local laws governing cannabis cultivation. It should also be noted that patients are allowed to have no more than 8 ounces of dried cannabis in their possession.