Emily - as an avid MTBer, and an avid SSer, I can safely say that a FS SS MTB is nearly impossible. Having broken derailleurs while in the woods, and needing to quickly do a SS conversion just to get home, the issue with pivots and suspension changing the chain line causes significant issues (as in, you can't keep the chain on) - just as Spokewench said.

I have a HT SS 29er with a suspension fork that I love. I don't find the lack of rear suspension (and I only ride FS MTBs) to be an issue for me, even on the boniest singletrack. I would suggest this as an option to begin with, and then think about the suspension seatpost as others have suggested.

Can't help with a specific brand as I'm significantly taller than you. DH picked up a Motobecane frame to build my SS MTB bike for relatively short $. I have a shorter friend who rides a Surly as her SS on the roads, while I ride a Kona SS-specific CX frame on the roads.

Good luck!

SheFly