Johnson's Seeds do catnip seeds at £1.99 a packet, and you do get a fair bit in them. You should be able to find them in garden centres and hardware shops. I had enough to give me about 150 plants from two packets (they were BOGOF!), most of which have gone to my local branch of CP for their stall and are apparently selling like the proverbial hotcakes. I've kept a dozen plants for my own use and passed some onto friends as well.
Catnip seeds take around a month to come up. Sprinkle them on the top of a pot of compost and then cover with a thin layer of the same - don't plant them too deeply. Catnip needs to be propagated in a warm place as if it's too cold, they won't come up. So a warm windowsill or a sunny patio is a MUST, and it helps to cover the pot or seed tray with a sheet of glass or perspex to keep the heat in i.e. minature greenhouse. If you've got a heated propagator, then use that. I have a heated greenhouse, mind...
Seedlings need to be kept moist, but not wet. Once they've sprouted, they'll get going pretty quickly - the plants I *did* get from my first sowing are already flowering! Ideally you do want to sow the seeds around March time, I think it's probably a little on the late side now.
However once you've got catnip plants, they can be propagated further by either root cuttings or layering.
