Hello Bob.
Thank you for posing a very relevant question. Â
Here in the UK we get our supplies through the NHS, which makes them free at the point of delivery ( However, we actually pay for them through a lifelong National Insurance Scheme)!
That aside, having exited the European Union ( where many of our medical supplies come from) we are already experiencing some of the difficulties you envisage in your question.
Thank you also for the mention of my efforts in making my own devices. There is something in me that looks at this stuff and  makes me feel that I could surely do a bit better with a bit of thought and common-sense.
The fact that I have already resolved some of the basic problems we might face in such a scenario, I hope you don’t mind me chipping in with some comments.
The first comment would be that anyone who can irrigate should practice that technique a.s.a.p, as this may resolve many of the issues that might otherwise arise.
Whilst I am still experimenting with all sorts of different approaches, my experience and logic tells me that the main problems now, and in that possible future scenario, would be with the baseplates.
All of my designs take ‘normal’ bags/sleeves etc, but would be very easily adapted to plastic bags of any description.
I actually prefer to make my ‘bags’ from irrigation sleeves because some of them are just right to fold up ands seal into a bag shape. However, if these were not available my first port of call would be to the supermarket, where there are usually innumerable types of plastic bags on sale.
My second option would be to get a roll of tubular plastic and simply cut my ‘bags’ to size.
I already have overcome the problem of sticking the bags to my base plates by using double sided sticky tape. At first this was straightforward carpet tape, which worked fine. However, more recently I have purchased a roll of 2.5inch wide double sided tape called ‘sookwang’ , which is made in Korea but marketed worldwide.
The 2.5 inches allow me to easily cut off the right size, stick it on the baseplate and cut the hole once the tape is stuck.
I used to use Hollister 7730 adhesive spray – but as they no longer manufacture this, it left us in much the same position, having no suppliers., as you describe in your post. Â
Without giving a lengthy description of the many baseplates and ‘bags’ I have made and discarded,
I can say that making one’s own has the great advantage of  ‘freedom’ from the constraints of the one-size-fits-all concept. My devices fit much more precisely and can be easily adapted as and when circumstances change.
I would not welcome the ‘doomsday/gloomsday’  scenario you pose, but feel that I am well prepared for such an event should it ever happen.
One of the more ‘fun’ experiments I tried some time back was with a condom. I did post a picture of it, but took it down after a while as It was probably not to everyone’s taste.  The only problem was that it blew up like a long, thin balloon when there was gas output. However, it did catch the liquid and solid output quite effectively.  Â
I have also used dog-poo bags, zip-seal freezer bags and found that the thing they all have in common is that they need a ‘pipe’ to guide the output right down into the bag rather than letting it pancake around the base of the stoma. My (effective) solution to this has been to use a 90 degree plumber’s waste bend but that’s another story!
I do hope there are lots of others out there who have experimented for themselves and I also hope that they may freely share their thoughts with us on what could , one day, be an important issue.
Best wishes
Bill