The Cloth Habit Watson (non wired) bra
@Celia @Flobear There is a sewalong on the website and a lot of info about fabrics and elastics.
I would have recommended getting a bra toile pack from Sewing Chest but they dont seem to have them at the moment.
If you get a kit to start it will come with all the bits you need though I do find the strap elastic a bit stingy. Sewing Chest usually have loads of kit stuff but it seems to be down to one page at the moment.
Also, dont get carried away by your rtw size. Start with the sizing guide with the pattern - it will probably be completely different from what you usually buy.
Do have a go - it's quite a revelation to have a bra that actually fits properly.