I have had a couple of these butterfly stamps in my wallet for years, not wanting to use them, just looking at them once in a while. Now I bought a few more, and a couple of people got them on their envelope.
I bought whole sheets (50 stamps) of the white flower and the church stamps. Most envelopes got two church stamps, one flower and one other random stamp to make it add up to the 10 kr value it needed.
I was a little worried that the mail would get there, but to my knowledge, all the letters arrived, so this is great way to add some extra personality to your correspondence!
For American stamps, check Etsy shops like Verde Studio or Love the Postage. There are plenty of others, just search for "unused postage". You can also buy unused stamps on eBay.
For Norwegian "postfriske frimerker", I used QXL.no. It is an auction site, so you can get lucky and get a really good deal, spending less on postage than if you bought brand new ones from the post office! It is a lot harder to find cute ones here than on Etsy, so plan to spend some time searching.