I think most vendors use child-proof caps, lots of them carry other supplies and tools for the DIY-er..have you contacted any vendors listed under ECF's "Forum Suppliers", in the bar on top of this page; a fun thing to do is to open the Supplier's list and just start going down through them all. Some sell only e-cigs and relevant cartridges, etc., some are more widely inventoried. All are very helpful if you have questions.
Also go the the Forum Thread "DIY" there are some suppliers mentioned outside forum.