This place was such a gem to find, but I would have NO idea how to explain how to find it. We just happened to be driving down some random highway on our way to El Yunque (coming up from the East/South, so you'd be passing El Yunque if you're coming from San Juan) and I saw "health food store" and had to stop.
I was disappointed that they didn't have any sandwiches or ready-to-eat items because that's what I was looking for, but they were the only place that I have found soy ice cream in Puerto Rico so I had to get a Tofutti neopolitan ice cream bar. I also got a bag of guava bakery that's made in the island and sold in many locations. We stocked up on some nut/fruit bars, which had interesting combinations like macadamia nut and passion fruit.
The prices here were reasonable, taking into account that islands always have to price things higher because of the cost of shipping. The big jars of grapeseed Veganaise were about $9.50 and the tubs of Earth Balance were around $6, so not completely terrible, especially since they're so hard to find in PR.