Hi Flugie
You have a very nice 2.3/4hp Douglas. Unfortunately you are very unlikely to find cables to fit off the shelf. The reason for this is that Douglas originally used a variety of carburettors of good make and also a variety of handlebar levers. So the result is that cable have to be made up to suit. There are two ways of getting replacement cables. Either send an existing cable as pattern to a commercial maker of cables or buy the materials from a supplier which is the most common route. In England we have a supplier who will make to pattern and who not only operates a very good mail order service but also attends most of the auto-jumbles (swap meets) throughout the year. From him you can purchase a variety of gauges of inner wires and outer sleeves and a full range of end nipples and adjusters to enable replication of any original cable using simple hand tools and a soldering iron. I am sure that there must be similar suppliers in The USA.
Replica manuals and Illustrated Parts Lists for 2.3/4hp models are regularly advertised as "buy me now" items on ebaY from the USA. Original items also appear from time to time as auction items but tend to now be very expensive. The London Douglas Club is now producing updated and replica literature for post-war machines and has plans to also produce replica literature for the more common pre-war models which of course includes the 2.3/4hp of which there are more survivors than any other model. Chris.