Can't seem to find a definite answer anywhere. What stores are stocking the new Core Set? Any online retailers? Anywhere outside the U.S.?

Target in the US is so far the only confirmed retailer. People in the UK are claiming that it is not available anywhere and the article posted by FFG says it's only in 'select stores' with promises of a wider release 'later this month'

Only US Target stores confirmed so far. Sunrise may bring more answers.

At my house.

Target only, at the moment.

Distributors will make it available for order starting tomorrow, although they cannot ship until 9/16 (Target apparently bought a few weeks of exclusivity), so your FLGS will get it 9/16 + x days, where x days is how long it usually takes them to get something from a distributor.

It's being recommended, if you plan to buy from your FLGS, to pre-order it rather than hoping to pick it up off the shelf. Supplies are rumored to be limited, so stores are likely not going to have more on hand than needed to fulfill pre-orders for a while.

Miniature Market now has it available for pre-order.