Finding cool interchanges in cities is relatively easy. Look at a map to get a basic bearing, then drive along the freeway until you see causeways going in all different directions. Once you get to a promising spot, get out, and hike up all the near buildings/hills/trees you can find. If hiking fails, the semi-affordable fallback is a heavy duty remote controlled helicopter that you can pick up for $299. You can even secretly buy it for yourself by giving it to your kid (I'm not saying I did that...)
Here I snuck into an apartment building and cracked open the window in the stairwell towards the top. I say towards the top, because I think the top is a little too obvious.