There are only a few retail outlets for the specialist materials needed to make a bra, and many of them are in America.
If you start with Beverley Johnson's Craftsy course on bra making everything she refers to is available from her shop Bra makers Supply in Canada
https://braandcorsetsupplies.com/#shop Many of the products are also available from B-Wear in Sweden
http://www.bwear.se/en/patterns/patterns/ and recently Fit2Sew in UK
http://fit2sew.co.uk/shop/has started stocking the same products.
My red bra
http://thesewingplace.org.uk/index.php?action=gallery;sa=view;pic=11 was made with a kit from B-Wear. It included more than enough fabric, channelling and band elastic but the strap elastic was quite short; they assumed you would be making the front part of the straps with fabric, which I wasn't planing to do, so I substituted different red strap elastic. Also the red lace was IMO a very poor colour match so I didn't use it with the rest of the kit. I did use it as a contrast with black
http://thesewingplace.org.uk/index.php?action=gallery;sa=view;pic=14 and I am glad I didn't use it with the red duoplex because with washing and wearing the red lace is now mid pink!
Another source of kits is Merckwaerdigh in The Netherlands.
https://www.etsy.com/shop/merckwaerdigh. My second bra was made from one of her kits, and although it looked nice the facric was a type of scuba and it made me really itchy and I couldn't wear it
even after a couple of washes. I am not usually allergic to fabrics. I still have bits of a second kit (with lace and tricot fabrics) from there in stash. Lovely bright colours but the elastics are all a bit flimsy and I didn't like the wires which are flat instead of the more usual round cross section.
Most of the fabrics, elastics and hardware I have used has come from The Sewing Chest.
http://sewingchest.co.uk/index.php?cPath=91. The main criticism of there is that unless you want to stick with plain black or plain white it is unlikely that you will be able to find all the required elastics in matching colours. My black and red bra
http://thesewingplace.org.uk/index.php?action=gallery;sa=view;pic=15 and my black and gold bra
http://thesewingplace.org.uk/index.php?action=gallery;sa=view;pic=13 are both made (mostly) from materials from Sewing Chest. Both have upper cups made from embroidered edging which has a mirrored design so that the cups match. My burgandy bra
http://thesewingplace.org.uk/index.php?action=gallery;sa=view;pic=12 has black wings because they didn't have any bugandy power net or substitute. I also made knickers to match using lightweight powernet at the back and that really hasn't worn well, there are bits of elastane fibre sticking out all along the edges.
I have bought elastics from The Bra Shop in Liverpool
https://www.etsy.com/uk/shop/thebrashop and found they were very quick to post, but I haven't bought fabric or a kit from there.
An online source of bra patterns is Orange
https://shop.orange-lingerie.com/collections/all. I have used their Marlborough
http://thesewingplace.org.uk/index.php?action=gallery;sa=view;pic=14 pattern a couple of times. Because bra pieces are small, even with the bigger sizes, it's not usually necessary to join any bits of .pdf patterns.
VeryKerryBerry
http://verykerryberry.blogspot.co.uk/p/uk-bra-supply-shops.html has a list of other suppliers, but as I haven't used them I can't comment.
A couple of other useful websites with bra-making info are:
http://www.foundationsrevealed.com/index-of-articles/bra-making/how-to-make-a-bra/161-bras-determining-your-sizeand
http://learning.makebra.com/instructions/Edited to make the links work properly!