Tip: try getting the fishies either ridicuously dirt cheap or free... Makes it easier =D And don't limit yourself to just store fish... owned and unwanted fish too
The one person who saw what I did, told me she went out and got her betta all new and upgraded stuff (hoorah lol). =D
Anyways, Kijiji, or craigslist... Local pick up only. Try making posters, flyers, talking to people (word of mouth works great, 6 of my bettas were adopted within a week because of it!), even make yourself a quaint little website showing the bettas you own, you rescued, and ones you rehomed.