a few nurseries sell the plants but they are easy enough from seed thompson and morgan sell it.After starting the seeds on a windowsill in a closed room I used to grow mine under an upturned hanging basket well pegged down with tent pegs The local cats used to queue up for it and it got big enough with protection to pick some for drying.
http://www.gumtree.co...ree-postage/103558479
Make sure it is definitely Nepeta cataria though as the other catnips don't have the active drug for ***
its supposed to be very good for them though our cats lived to 18, 19,and nearly 20 and my mums to 21 and 23 and they had it regularly. I think it helps with their aches and pains as they get older as they tended to just eat it and go to sleep.