I hate that feeling, I've been there! There's a walmart about 45 minutes away from me that I was in over the summer, they hadn't been keeping many bettas in stock and when I went in there were only 4 bettas there, 2 girls, 1 live boy and 1 dead boy... I took the 3 live bettas that were left. Went back about 2 weeks later and they had about 30-40 bettas, the most they had had in months, and just like you I couldn't help think "if I hadn't bought those last fish..." but I know its not my fault, as Zubinbetta said the purchase of the fish was a drop in a very big bucket.
One hand reaches out and pulls a lost soul from harm. While a thousand more go unspoken for, they say what good have you done? By saving just this one... It's like whispering a prayer in the fury of a storm. And I hear them saying... You'll never change things...
MoonShadow Betta Fish Rescue
Saving the world one innocent life at a time