Yes, your idea of 7-8" is ok for a larger plant like a 1 gallon. 8" might be better, giving you room to get fingers in to remove the plant when needed. You don't want your plant sitting too low in the outer pot. If I have this issue, I usually get something like a tuna can to put in there to boost it up. You can use anything. Not sure about rocks. Don't buy a tiny plant and put it in a 6" pot. Plants don't thrive that way.

Another thing you do not want to do is water your plants while they sit in the cache pot. The water will collect on the bottom of the outer pot and won't be good for the plant. It might stay too wet. Water in the sink, outside or wherever, and let the plant drain overnight, then put it back. By the way if you are doing succulents this way in cache pots, they really don't do their best in cache pots. FYI

I only order succulents through the mail, so I don't have a suggestion for you as for a vendor. In my part of the world we will have heat through end of October, so not sure how successful shipping would be. If you find a vendor, you might ask this question, how they survive shipping. Home Depot might have pots.

Have fun, don't go hog wild! What plants are you thinking of? Pothos is probably the easiest, peace lily too (spathiphyllum).

@socks Thank you! I decide on an 8" as well, I'm flabbergasted that there don't seem to be a lot of options online for bigger cachepots. The average size available seem to be 5-6". I'm expecting 2 plants in 6" pots as of now.

Pothos and the peace lily were on my list, however, I wanted tall plants to fill up some odd spaces at home (we have our TV in front of an old fireplace) and I have a cat (turns out they are toxic to them). I've ordered Palms as I wanted tall bushy house plants indoors (I'm in love with the broadleaf lady palm). For our patio, I'm looking at the purple princess (Tibouchina urvilleana) and the canna lily, for a touch of color (and again, looking at tall plants, to provide a bit of privacy as well)

It's a bit more hard to get my hands on live plants as opposed to acquiring bulbs, so I'm at the mercy of online stores right now, in terms of which plants are available. I shall look into the nearest Home Depot as per your suggestion as well. I feel like I can make a better informed decision with an online purchase at this point, as I don't know enough about plants and I have to cross check to see if they are cat safe.

You seem to have good knowledge about plants, so you probably know of the sanseveria or mother-in-laws tongue/snake plant. They grow tallish and manage with low light quite well. I also have a rubber plant outside which will grow taller than the house if I don't whack it back a couple times a year.

I get the idea that you do not wish to go in stores, and I understand that, having my first grocery store trip today in 6 months where I bought two plants (groceries too). I do think you'll get more choices and better prices if you can get out. Baskets can work too if you put something in the bottom to protect the surface it's on. Lighter and cheaper. Have fun!

PS you can make plants taller by putting them on something, stack of books, upside down pot, small table or plant stand, anything wide enough to keep the plant stable.

Remember that a cachepot doesn't have to be sold as a cachepot. You can use other things depending on the size and shape of the plant, like kitchen utensil crocks, wastebaskets, and deep bowls. Galvanized pails or kitchen stock pots if you want the metal look. Anything the inner pot fits into that holds water, or that will hold both the inner pot and a plastic saucer.

They have large cachepots at Ikea. Plain white. If that's your thing.

Like Edie said, you can use all sorts of things as cachepots. Tons of options out there if you think outside the box. Even a box! haha.

I particularly love vintage pottery. Mostly varying shades of green. I don't like to plant directly in them whether or not they have a drain hole. So I use them like cachepots.

If you are using cache pots for outdoor plants they’ll need a drainage hole, unlike indoor cache pots. We don’t know your location but your two outdoor choices aren’t hardy in cold winters. And the canna will die back to the roots.

If you go the Ebay route, look for a seller with many positive reviews. I've bought plants and other items and it's always worked out fine, even the weird items like live roaches to feed our pet bearded dragon. I always check reviews before ordering. Buy from someone with mixed reviews at your own risk; I've heard plenty of unhappy stories. You can also check Goodwill online if you want to thrift from home. Goodwill Hunting