I fancy playing around with this but finding it hard to find official ones.

Would this be ok?

http://www.ebay.co.uk/itm/151747032805?rmvSB=true

Or, can you recommend me one? Thanks

Should be fine - it doesn't matter if it's an "official" one or not, really, because it's just a bit of cardboard and a couple of lenses; the phone itself does all of the 3D work. I had a cheapo one off Amazon until I upgraded to a Nexus 6 and it no longer fitted, and it worked great.