Pineapple Fields from Professor P is the shit. He does alot of modern work with sativas, and preservation work with bx-ing old stuff.
nl5/haze (and all its variants) are going to be the bulk of the lineage behind "short" flowering sativas on the market in my experience.
We are talking SSH (technically nl5/haze x slunk1), Cough, piff, Kali mist, Consumption, dog shit, etc... real blue dream is hard to find in seed form, but it is pretty great.
Short flowering sativa hybdrids will still run 12+ weeks for the best expressions, and stretch will occur, so he should start flower early.
Professor P and AKBB have the best selection in my experience. I'm trying some gear from another sativa breeder now, but I don't want to plug them until I grow their shit.
Here's some PF from prof p
View attachment 5354979
Here's some Consumption x Choc thai (running now) from akbb
View attachment 5354988