Not the earliest example, but... I know that in by the high middle ages, pilgrimage routes in western Europe had solid souvenir trades. Not only were religious pilgrims keen to identify themselves with [badges] (http://collections.museumoflondon.org.uk/Online/group.aspx?g=group-19998) to show they were part of a pilgrimage, there also seems to have been a functioning trade in [small tokens] (https://books.google.com/books?id=x0s9XuWbUMoC&lpg=PA124&ots=5wON1iXx64&dq=medieval%20pilgrimage%20souvenirs&pg=PA124#v=onepage&q=medieval%20pilgrimage%20souvenirs&f=false) commemorate the experience.

There's actually quite a bit of ink spilled on methods and manners of commemoration in the middle ages that specifically relates to souvenirs and the like. This catalogue is from an exhibition of such objects. Netzer (one of the authors) has argued (to me, in person, therefore no citation) that pilgrimage souvenirs should not be separated out from other material culture of the period because they were made for "tourists". She believes, rather, that all of the material culture examples from the period should be together understood as commemorative and memorial. Even if the object were made for export, it is a sort of representative missive intended to allow a piece of the producer (individual or community) to travel. So, she has argued (again to me) that all portable material culture is a sort of souvenir,even if it's high status.

I actually disagree with her in this respect, and think it's an overextension of a well-intentioned theory. But i wanted to include it nevertheless for your consideration if you continue to research the subject.