I got mine by post from Hindle Power, Sturrock Way, Bretton, Peterborough (home of Perkins) PE3 8YJ, England. Tel: 01733 334300 Fax 01733 334200.
Give them a ring first (they are REALLY helpful) as some (such as the 4108 marine version) are printed to order. Mine cost £30 and has been worth every penny. Invest in some A4 size covers as well, as the manuals are loose leaf and it protects your investment from oily fingers!
Its the same manual. The 4108 manual applies to the 4107 and 4.99 engines as well (says so on the first page). So you can definitly get it from Hindle/Perkins at Ptereborough - they will post it out to you. Any problems, send me a private message and I'll sort one out for you from this end (UK).