I have also bought from Fullmoon multiple times. She provides a good variety of quality product for both nail deco and regular decoden + other stuff, and I do recommend it.
I've also bought from Yukiumi before with no problems.
Probably the best place I found to search for cabochons (smaller and bigger pieces) is Etsy. Looking at my spreadsheet, I've bought from (Etsy seller names) Blingers, happybaby8222, misssapporo, RoppongiKitsch, and SoiFlower.
My favorite seller of them was misssapporo. Good quality items and good selection. That's where I found the My Melody heads (among other great pieces).
Blingers for rhinestones. They're not Swarovski-quality, but they're better than the super cheapy eBay ones.
happybaby8222 (it's like maybe a B tbh), but the prices were good.
SoiFlower so to me, that means that the transaction went well.
Other sellers that you might want to consider - MimiLoLo, delishbeads, PixieKhloe and SophieToffeeCo.
For eBay and Etsy, my advice is to take some time to search around for a bit because it is possible that there are multiple sellers listing the same or very similar items but for different prices. If you want to keep your total down, a little searching can go a long way sometimes. Also, please be sure to look at the measurements for each item because 7mm items can actually be pretty big for nails.
I've only attempted to do a decoden phone case once, and it did not turn out very well lol.
Tbh, if you wanted any of my decoden stuff, I'd probably sell it to you since I'm not using any of it really lol. Not the nail deco items though because I find them much more useful haha.
If you're interested in the cases I keep my items in -
My dad got the bigger clear plastic container with blue snaps from a fishing supply store.
I bought the smaller clear plastic case with purple snaps from Wal Mart in the sewing section.
The cloudy 8 compartment (4/4) pill cases are from Etsy (seller CuppiecakeSprinkles).
The longer 12 compartment (6/6) cases are from eBay and some of the compartments open too easily and others are impossible to open [in the second picture of this post but not seen above].
The crystal clear 10 compartment (5/5) cases were a gift bought in Japantown in SF specifically advertised for deco pieces.
The pink/purple caboodle I've had since I was a child and I'm not sure where you can get one now.