In the United States it would be at the USPS web site. They will deliver them to you through the mail.
* * * * * * Stamp catalog (postage stamps) * * * * * * * * * *
There was never a 12 cent stamp for regular postage. It went from 10 cents to 13 cents on 12/31/1975. There was however a postage card stamp for 12 cents in 1981.
First class postage was 29 cents that year. A post card would be 19 cents.
In April 2010 a first class stamp was 44 cents. A post card was 28 cents.
First class postage was 15 cents for the first ounce in 1980. A post card cost 10 cents.
First class postage was 41 cents for the first ounce in 1955. A post card cost 26 cents.
First class postage was 3 cents for the first ounce in 1940. A post card cost 1 cent.
First class postage was 3 cents for the first ounce in 1956. A post card cost 2 cents.
The postage rate for a standard card was 32 cents after 22 January 2012.
Unless your card is larger than normal, it shouldn't cost any more than a first-class postage stamp to mail a Christmas card from the US to the UK.
In 1936 a first class stamp was 3 cents. A post card cost 2 cents to mail.
Postage in 1966 was 5 cents for a letter up to one ounce. Fora post card it was 4 cents.