Ok, I get several questions about this every week, so here we go:
1. Go to www.ebay.com (if you have a national ebay site like ebay.co.uk you can use that too).
2. Search for "nail art stamping plate".
3. Get a lot of results and start sorting through them.
By far most the Hello Kitty plates I have seen have come from Hong Kong and have been part of a set. Usually the price is quite resonable, but watch out for shipping costs. Not all sets will include the Hello Kitty plate. Make sure you make it VERY CLEAR to the seller that you want the Hello Kitty plate. The GCOOL Hello Kitty plate has the number A02, but sometimes it's not enough to give the number of the plate, since different sellers have different numbers. At times there is a language barrier, so be patient :)
Hope that helps and remember to peel the plastic of the plates when you get them!
4 hours ago