I'm in the states, I'm officially keeping my eyes open for them because for some reason they fascinate me now.
It depends on who you ask. Very traditional etiquette mavens (such as I think Miss Manners) will still assert that response cards are inappropriate - they are an implication that the invitees are so etiquette-challenged that they do not realize that they are expected to respond, in writing, on their own personal stationery.
I am etiquette challenged! I don't even have personal stationery. I mean...I have some notebook paper with unicorns on it....but I'm pretty sure that's not the same.
I'm officially going to ask my grandma what she would do if she got a wedding invite without a response card (or one with a response card). She has stationery without unicorns on it, so I bet she'd know to sit down and write out a response on her stationary.
Vistaprint is your friend 
I order heaps of stuff from there - I have personal stationary, address stickers and even personal stationary for my dogs also!
You have personal stationary for the dogs? You win. At everything. Constantly. :-)
LOL. The stuff at Vistaprint is honestly so cheap and the quality is pretty good too. I figure if I'm going to spend money buying a notepad anyway, I may as well get it from there and have a picture of my dogs on it

. I use it to write cute little notes sometimes to my husband like "Dad, don't forget Mum is working late tonight, so you need to feed us. There is meat in the fridge which we will have with some pasta. Woof Woof, Dogs"
Once you order something from them, you get so many e-vouchers that you don't have to pay for shipping - just wait for one of the vouchers to come through and then place your order.
Anyway, going off track a bit for this thread - apologies
