I slant and trade, and recently mentioned the same thing in one of the clubs I belong too. Of maybe 100 people, 5 might have been seriously interested. Of those 5, maybe 2 or 3 I would trust to create a clean sample that I would risk a batch on, or re-slant to store. It takes a little more nerd-factor and cleanliness OCD than the basic brewer may want to deal with. It's not difficult or expensive, but to achieve long term storage and proper re-buildup, it needs to be done right. Cold storage agar slants are the best bet. Just putting trub from the fermenter in the fridge isn't going to remain viable for long periods.
My opinion is definitely do your own bank! Make 2 or 3 slants of each yeast you use. It can save money and headache in the long run. I started because I was having to buy a lot of old yeasts that required a hellova starter to bring back to life. I figured if I have to do life support starters all the time anyway, screw it, bank my own and stop paying for it.
Find others that bank their own and offer up a trade list. If someone is interested in it, teach them how to do it. But always remember the risks involved. You wont know when someone says "meh, it's clean enough. I dont need to sterilize that" and you wind up with an unexpected sour.
Sometimes when friends are brewing and using a yeast that I want to add to the bank, I ask them if i can steal a drop of their yeast when they first open the package. I bring over an inoculation loop, a slant and my alcohol burner and heist it