Wesley's are super rare, they come up now and then, check gunbroker or any other gun auction site but be prepared to pay a lot for any parts.
I'm no expert on Wesley's, but they have several flare launchers that utilize the same or similar base body, so you may be able to get a trigger guard or other small parts from a different webley flare launcher that will basically be the same, so check that.
When I was searching for them.,I found numerous webley flare launchers on gunbroker that were so similar to the mk 1, but with no shoulder stock and a short barrel, so it just depends on what you need vs what you can find.
Rafelfett offers extremely accurate 3d printing files and you can always have those parts printed ( in metal too by some online companies such as Sculpteo, for example) and they do such a fine job it would be an excellent compromise and/or substitute for thw real thing.
I have had several gears for other projects printed in metal from Sculpteo and they are amazing. Same as a cast steel part.