Helm is the publisher and retail outlet for Ford service manuals. helminc.com for information. Ford parts orders from Helm, I don't know if they do a markup. There are businesses offering to sell "official" manuals more cheaply but I don't know what they are selling.
There is a difference between service manuals and workshop manuals, the latter being more detailed and much more expensive, obtained usually by subscription.
There may be online sources for free downloads of factory service manuals or the workshop manuals. Try workshop-manuals.com, fordservicemanuals.org, or www.repairmanuals.com.
My levels of DIY have found Chilton or Haynes manuals adequate, but the closest you'll get to E-450 will be the Econoline or E-Series collectively over a rather broad range of model years.
