Online Circus Art Museum from the Netherlands
Pat Cashin (the curator and mastermind behind http://www.clownalley.net) asked me about a source for European circus posters.
Before I had clownlink, I had found this and emailed people about it, but apparently never thought to put it up online.
This is an amazing resource of thousands of images from European circuses.
And a lot of them are available for sale. Looks like prices range from 7 Euros to 105 Euros, and the purchasing engine is in Dutch only. But it looks pretty straightforward.