Just got back from my 2nd trip to Cavacini Landscaping & Supply (and Garden Center). (FYI the address here says 50th St, but I've always accessed it from 51st. St across from the Goodwill).
Over the winter, I bought my Christmas tree here; they had a good supply of small trees left when most other places had already run out.
Just now, I picked up some organic bagged potting soil (they also have soil & mulch in bulk; I think they said they do delivery too) and some vegetable and herb seedlings. Very good prices and decent selection. I wish they had more herb seedlings, but I know I'm a bit late in the season so maybe they've run out of things. They did have a number of varieties of tomato and pepper seedlings, as well as other veggies including cabbage, celery, okra and beets.
Both times I've come here, the staff has been friendly and helpful. They also mentioned that they're going to start selling local corn and tomatoes once they come into season next month! And they have plenty of parking, which makes loading your car nice and easy.
This is a really solid place that is overdue for a Yelp review!