i get it from pretty much everywhere; 8 speed chains have about twice the durability of the thinner 9 and 10 speed chains. Not a big issue, but i do like durability.
Chains are consumables when it comes to cycling. Personally, I would put chain life is the least relevant of criteria when it comes to choosing between 8 speed vs 9 speed or 10.
Sora's thumb shifters are not as nice to use as the dual action levers of Tiagra or higher. Being able to shift easily while in the drops >> chain life, IMO.
V.