all depends what you want your car to do for you. i personally just have a rear sway bar to help eliminate the body roll and understeer i got after lowering the car. i didnt really need the rest. worked wonders for sharp corners, like on-ramps.
i hear mixed things about front strut bars. some say they work to stiffen up the front, and some say they dont do much of anything. braces underneath (my guess, not experienced with them personally) will stiffen your chassis as a whole, but obviously doesnt interact with the suspension.
i got my bar from
http://shop.microimageonline.com/
its the ultra racing rear sway 19mm. most of the parts are also listed under the yaris, but we share the same suspension. they have a scion xD category tho. if you email them they get back really quick as well.