just a note re: silk vs. poly in the running/fading area. There are significant ballroom fabrics that run due to the dye process not being finished. I use silk all the time and have yet to have one of my silks run. I would test any silk you are planning to wash, but there is no way to know when you buy "real" ballroom fabric. Once the fabric is cut, thats it, no return. And its not really easy to do when most of it comes from Europe. I don't want to discourage anyone from working with these fabrics. as the benefits outway the issues, but you have to be prepared for it. The most wonderful fabric in the world is the dancecrepe and it is the worst offender. I have washed several crepe gowns only to find out they aren't fast and although its not my favorite thing to do, it is relatively easy to deal with if you aren't dealing with alot of different fabrics and colors.
Most domestic lycras and microfibers are fast as their original market is swimwear, so if you are buying from most of the aforementioned sites, you will probably be ok. But I will tell you there is a huge difference between a domestic lycra and a european lycra and an even bigger difference between a domestic poly chiffon and a euro poly chiffon.
I started sewing for myself and explored the same sources you have.....although we didn't have the internet:0. It was obviously a long time ago! We have ability to buy from anywhere in the world now and quality of fabric is directionally proportional to the quality of movement on the dancefloor.
I made my first competition dresses out of all silk. Silk taffeta, silk chiffon, silk organza, silk jersey - I thought you had to spend a lot of money on the fabric, because it was a "ball". OMG. I think I spent $1300 on fabric for 2 hideously weird dresses. The only poly on either of them was 40ft of gold lame ruffle on my latin dress. 20 years later, and I can honestly, and thankfully say, I have come a long way. But the next ones went too far in the other direction..........way too much cheap poly and spandex. Fortunately, I covered up all that cheap fabric with 8 ostrich boas. And 32 rhinestones. I am not kidding. I truly couldn't make it up. There are photos somewhere in this world to prove it. And my hair........it looked just like Katusha Wilders does now......I noticed this at Heritage........in an upside down ice cream cone. I thought I had invented that look and retired it forever.
So good luck!