My friend Lol555broek Beatty is the creator and owner of Celtica castle. Though Celtica is not a store (it’s more a place people can go to hang out or be romantic) if you click on things you’ll find various items for sale in a celtic or goth style. Lol555 enjoys putting secret things in her castle like phantom walls and poseballs; and if you’re part of her group you have access to even more of this stuff.
Lol555 has a store called Sensations as well. One section is underwater and she has 104 freebies in a vendor for you. On different floors she has furniture and art. Ignore the full perm stuff and just look at the stuff she’s made. Also, if you need a low prim skybox there are 8 prim ones for 50 lindens called Cubinisimo. With a click you can change wall and floor textures in these, so they are newbie friendly.
Visit Inworld: Celtica
Visit Inworld: Sensations