The cheapest route is to acquire a time machine and head back around 25years. This stuff has been almost impossible to get hold of for +15yrs (even small quantities).
Best bet it to just post a few Wanteds around the forums/social media. I know a few people that managed to get a few meters of it, but even that was probably a decade ago, and they've long used that up. To get enough to redo 4 door cards is going to cost a fair old whack. Even if you're going for concourse, it might actually work out cheaper to just find 4 mint complete door cards rather than sorting yours.

Its also worth phoning a couple of retrimmers. Someone might know someone who knows someone else that might have some - these things sometimes work out