Plenty of Science Fiction for the Kindle at Amazon.
http://www.amazon.com/s/ref=nb_sb_no...pera&x=20&y=16
I don't think Asimov's is losing subscribers so much as they are changing over to a Kindle format either. I have been getting the Kindle edition of Asimov's SF Magazine for the last few months for the sum of $2.99 (per month) which I'm sure is lower than the paper edition's cost. It just downloads to my Kindle as the month comes round.