Go to mamutphoto.com, specifically http://www.mamutphoto.com/content/view/92/43/lang,en/ , for a list of useful lenses (and a lot of other useful information). You will need to know the diagonals for the formats you are considering. Once you have worked out what you want then just post WTB ads here and at the Large Format Photography Forum (http://www.largeformatphotography.info/forum/index.php) and also scour eBay.