Ebay usually good if you want rubber stamps or try google shopping.
Buying a rubber stamp for one card is quite expensive though - id try to either just buy some thing like a post card, look for a second hand book to take an image out of or try to print something (you could always age it by tearing/scrunching and using teabags and coffee granules)
So brothers 21st. I have planned in my head a brown tent card with a layer of embossed cream or aged white (using cogs folder) then a scrap of aged newspaper print followed by the main image of train (brown on white layers) with twine and button. No idea about sentiment! How does that sound?! Or any ideas of cards with a steam train stamp?! Thanks