Hi ladies. My stepsister has a very big swayback - her waist just disappears to nothing in the back! I made her bridesmaid dress when our parents married (awwww....). It was a princess-seamed dress and the alteration was a breeze - just pin out any excess along the princess seaming in the back, and leave the front alone. Depending on your figure, you may need to take a little in on side seams.
As far as garments that would be easily altered. Anything with princess seams would be ideal (coats, dresses, tunics). Any garment with a center seam and two darts on the back on either side of CB would also be a good candidate. In fact, those darts can be added if they don't exist and the garment is unadorned in the back. Darts or pleats that release into vents would also work well. Just buy so the garment fits at front waist, hips, and rise - with just bagginess at the back above the bum.
Any garment with fussy details around the waist in back, like a sewn in belt, would be a pain to impossible to alter. (like the back of this jacket:
http://www.hotpatterns.com/hp-.....ay-jacket/ )
This coat, on the other hand, has a belt applied on top of princess seams, so it would be quite easy to alter:
http://www.hotpatterns.com/pro.....-Coat.html
HTH!!! steph