Agreed, not easy at all.
The better course of action might be to contact an established, well-known decal company, who already has a good distibution and distributor base, preferably world-wide, and make them aware of the gap in the market. This would need to be backed-up by evidence (eg, quoting those now defunct decal sheets once available), and by a demonstration of knowledge of the market and the decal producers (the latter are not neccessarily marketing their products, but printing for others, for example Microscale, and Cartograph), with an indication of the requirement for the product(s), demonstrated by examples of users, for instance, from this forum.
Guttorm's idea of notifying his contacts at Revell (Germany) is also another possibility, in creating the idea of a new product line, as an 'accessory', for a waiting market. Approached correctly, this could yield a response, especially if the 'range' of possible decals was kept simple, dealing with letters, numbers, and similar items for popular subjects from various air forces/periods, in the most popular scales.
Other than that, if no luck with the above, then it's down to me, and/or others like me, to take the risk of having, at first, a limited number of basic sheets produced, and hope that they can be 'moved'. Of course, this could work too well, where demand exceeds the capacity of a 'one man band', and requires a full-time business to operate the design, production, marketing, sales and distribution!