I have a pair of Aphrodite’s in XL, which gives me a 40E/42D. I love them – especially because they are totally sleep-safe. There are some less expensive forms with a self-adhesive back. I believe Amolux and Gold Seal are 2 brands that make them.
None of the self-adhering forms will stay on for long without a bra. With a bra they work well. If you want to go braless you’ll need to use stronger adhesives.
As others have mentioned, Hollister is readily available, reasonably priced, and produces a fairly strong bond with the skin. Personally I don’t like it very much. The spray goes everywhere and makes a mess. It’s wasteful. I also found the bond lacking for multi-day wear.
I eventually stumbled upon Telesis adhesive by PPI. Telesis is widely used in the film industry by special effects artists to attach all sorts of silicone appliances to actor’s bodies. It’s superior to anything else I’ve used. Properly applied, I can wear my large Aphrodite forms continuously for 3-5 days, depending on environmental conditions. Going braless, bathing/showering, and swimming are all possible. Telesis is a bit pricey but a little goes a long way. A 1-ounce bottle will last months. There’s almost no waste since it’s a liquid you brush on. It’s available from many theatrical/film supply stores.
All medical adhesives are a PITA to remove from your forms. It’s best to tackle the sticky mess after each wearing.