Yes, you can probably find those brands at Nordstrom's. I live closer to Bloomie's so I can't be sure.
Dress pants are a bit trickier - I often end up with saggy butt too. I think dress pants, however, can be tailored a lot easier than jeans can. There's no yoke to deal with, etc. So this is definetly worth looking into.
Otherwise, I will say I have the *best* luck finding dress pants at banana republic. Try their Martin style slacks. No saggy butt with those!
Only other tip is to also pay attention to the back rise measurement. If you have a pair of dress slacks that doesn't give you saggy butt, measure the back rise. Unfortunately, online sites rarely give this measurement -they only give the front rise. Some will provide if you ask nicely.
When I was on my mission to find the perfect jeans, I discovered that back rise was just as important, and back rise is all over the map!!! You can have jeans with an 8 in front rise and the back rise could be anywhere from 9 to 16 inches. Huge difference! If you don't have much in back, you don't need the extra inches because you won't be filling it out.... and the excess will just ... sag. You probably just need a back rise that's only slightly higher than the front.
Since everybody is different, what I suggest is that you take a pair of slacks in to the tailors, and ask a lot of questions about what they alter to make the slacks less saggy for you. Then that will give you an idea of what kind of pants will best suit you.