I've looked in my local Home Depot, lowes, and another small garden store but have yet to find a an umbrella plant. Maybe it is because I live in Texas, but what places have y'all gotten umbrella plants from?
Usually the places you went to should have them. If a nursery doesn't have them I don't know what to say because they are very common office plants. I see them everywhere.
Maybe it's just Texas.
Try another nursery, or if you guys have a Target store if they have a nursery (sometimes they do) they might have one.
Home Depot and Lowes keep their tropical plants inside. I remember my first time going to buy some plants... ahh *nostalgia*
Back to the subject. It is a pretty common plant. It will go under the name Schefflera Arboricola, or just Schefflera. Its not in the garden section, its inside the store next to the garden section.
Found mine in the indoor plant section at Home Depot, it was darn near buried behind a bunch of palms. They had it in a tree form. I was so happy I bought it without measuring it, lol. It's going to be to tall for the cage by a foot. Maybe I can use it for free ranging, or if it lasts, it'll work when I upgrade to the next size cage.