I'm not 100% certain but i think you can create an soe account and just subscribe from that then download the planetside client
Edit::
Just checked
You can create a free account at soe,com then if you go to free to play games an scroll down you can see planet and purchase it for £12.99 which will include a 30day subscription. Once thats done go to this address to download the client
http://planetside.station.sony.com/download.vm