I usually shop around online and buy from whomever has the least expensive price for the Grodan grow cubes. Sometimes it's Amazon (as much as I dislike using them), sometimes it's not. Don't forget to compare total price including shipping, though usually most places have free shipping over a certain amount, and the cost of the rockwool cubes in bulk is typically enough to qualify.
For example, right now I see a place called HTG Supply has this (
https://www.htgsupply.com/products/grodan-grow-cubes-5-cubic-feet-loose-in-box/) for $110.
how long do they actually last? What do you do with the spent media? Are you using straight rockwool or adding leca/pumice? Any drawbacks or problems specific to this media?
Grow cubes last for years. They are inorganic, so they don't really decompose, but over time they do start to break apart and turn into dust. If using them in a completely inorganic media, you can probably go several years or longer between repotting (with caveats). However, in a mixed organic/inorganic media, you might not want to push it past 2.
Spent media goes into my compost bin. Sometimes I reuse it for non-orchids, by mixing it in with soil and then potting up my succulents or bromeliads.
I generally don't use straight rockwool, but some folks do. I usually at least mix in some large grade spongerock. I also frequently use it with bark based mixes.
As for drawbacks/problems, you should definitely soak it first, then check the pH and conductivity of the water to make sure it's "clean" and that the pH is appropriate. Otherwise, you'll need to soak and rinse, as well as use additives to get the pH to something reasonable (i.e. citric acid if too basic, lime if too acidic). You may also need to soak it in a weak fertilizer bath before use. Rinsing also reduces the amount of dust particulates. And on that note, you should definitely avoid inhaling the particulates, so use it in a well ventilated area, outside, and/or use an N95 mask.
Rockwool is a sludge, algae, and invasive fern magnet. It's hard to keep it looking clean and fresh in situations with decent lighting and moist conditions. The sludge and algae look gross and can reduce air flow into the pot. The ferns are mostly a nuisance unless they get really out of hand, but if you don't have those in your collection already then it probably won't be a problem.
Figuring out the feeding rate can be a bit of trial and error. Too little and the plants will develop nutritional issues because the rockwool doesn't provide anything on its own. Too much and the excess builds up quickly in rockwool and can form a crust on the top that's hard to get rid of.
