Not long ago I could get it at Walmart, their brand was $4.77 for what seems like forever.
But the last 2 times I've had to buy ATF, whether+4 or dexron, prices have gone stupid. Last time I needed some +4 I had to go to like 3 different places to get enough to do a pan drop/fluid and filter job, and $10/quart was at the low end of what I had to pay.
Used to be I could fill a newly built trans and converter for $50. Now a days I can't do a pan drop for that.