I think we've been to every nursery on the south coast, my husband is always planting some sort of tree.
The only jardineria I can think of right in the city that I can give directions for is on Independencia, heading out of the city going west. Pass the Presidente factory, go another couple of miles, it's on a corner on the right side. I look for the red Mr. Movie sign [also on the right] to know it's getting close [if they ever take that sign down I'll get lost].
They have lots of different plants there, I'm sure you'll find what you're looking for.
My husband just came in and he said to go first to 'ciudad ganadera' on the malecon [Ave George Washington], right near the Presidente factory, past the Leon building. It's also a mercado where you can buy fruit and vegetables, but he says they also have a nice jardineria there with very nice plants.
Then if you don't find what you want, you can go one block up to Independencia, make a left on Independencia, and go the the first one I mentioned up top of this message.
The nursery I like for value / selection is near the carrefour store on Duarte (highway that takes you to Santiago). You come to the stoplight just before Carrefour and make a right, drive straight on that road for about 200 yards, and it is on the left corner (the road gets a little bumpy just before you arrive).