People selling cheap umbrellas pop up on sidewalks like mushrooms the instant it starts raining, in any large city around the world. The umbrellas sold there are generally crap, but will work for a few hours if you're careful.
Honestly - I think umbrellas are only marginally useful. OK for ducking between one doorway and another - just a quick dash to minimize how wet you'll get. If you will be walking around in a rainy place, get a good hooded waterproof rain jacket. You get can a good-enough one that's small, light, effective and cheap at a place like REI or any decent outdoor store.
I never go on any trip without 1) a small, lightweight - but 100% waterproof - hooded rain jacket, and 2) a good rain hat that will keep rain/drizzle off my head and face (keeping my glasses dry). These items are tiny (the jacked packs down to a baseball-sized lump, and the hat is floppy, light and small). If I'm going someplace where I expect to be outside much (which is pretty much everyplace I go) and if there's much chance of significant rain, I usually bring waterproof rain pants, too.
If you have some distance to cover, and it's raining hard, an umbrella will NOT keep you and your things dry - they are utterly useless if there's any wind along with the rain. Believe me! Hey, I live in Seattle, a city that's notorious for rain. Question: How can you tell the difference between someone who has lived in Seattle for a while, and someone who just arrived? Answer: Look outside on any rainy day...the newcomers are easy to spot - they're all the ones with umbrellas (the locals are dressed in Goretex outer-ware).
YMMV. Good luck.