I went for the local shops first, and like you said, Joann does have some alternatives, but they generally do not carry specific types of fabrics like ballistic nylon or certain grades of muslin etc. After my trip to Joann, i didnt want to start my vest with the wrong fabric so i hunted fabric stores online. The problem with that is that you get a few "stores" that are less than legit, and you will get hits on the exact fabrics you need, but when you go to the site you will find it tends to be a bit shady in their listings, or you will order and never get what you paid for, or the listing is not exactly as described and you cannot return. So take extra time and make sure you are buying from who you think you are buying from.
I did find all the correct nylons for the vest, and i found a lot of variety of the orange flightsuit fabric but the colors range wildly even when they are all listed and shown as looking the same. Ordering online is a major hassle with specific fabrics. And i did not just do one method of looking. I used everything from ebay, to etsy, search engines, hunting at specific dealers, phone calls to specific dealers, i sent emails, i talked to RL and 501 friends, etc.. looked for remnants or others needing the same to order a bolt.. i tried it all.
The RedKap fabric is basically very thin. You have to be careful working with it. It will rip, and it will fray. It gets messy. But it does sew nicely, and i am going to use the rips for damage when i get the suit done. I have had to make some major modifications to the suit to get it closer to screen accuracy and i have not seen any threads that show all the process, only a pattern here or there, or a suggestion of what to use. And no one mentioned the pockets and seamlines that the RedKap needs to have fixed.
On another note. I do not care about 100% screen accuracy for any particular movie. I like it when its achievable over time, but not when you are 95% there and there is one detail someone points out keeping you from enjoying what you have made. Having spent 15 years in the Legion, i saw a ton of calls and gripes on peoples costumes because one small thing did not meet the standard, just to have that detail changed the next time a movie or show came out, or a new BTS photo surfaced that made it acceptable much later. I have seen a lot of people leave or be shunned by the leadership because they didnt get it exactly perfect at a given time. They were not "approved" for wear or accepted for membership because of a detail no one would ever notice except upon micro-inspection. Instead of saying "welcome to the club but keep working on this so its better and we will help". So having a 95% accurate costume is completely acceptable to me especially when you are in the process of creation, or you do not have a piece you need for completion, or you created your own character and there is a story behind the difference, or you are on a budget and you made an "executive decision" about your particular costume until you can make the time or funds to get it right.
My RedKap suit will not be 100% accurate because of one seam. You may not notice it when you see it. You will be told about it when i do it here. But i am not needing this suit to be "approvable" by any group. Its for my enjoyment and for helping others get enjoyment out of it. There is plenty of great costuming out there that is 95% but not part of a group. If you want to be part of the group, then just fix this seam and you are perfect.
I need to take more photos today. I realized i did a lot of work over the weekend and didn't really document much of it at all. Taking photos of orange on orange makes for an extremely muddy photo. You can almost tell what you are looking at.

But i will return today with some photos and a new post. Which i was not able to do last night because of some extenuating circumstances. And actually, tonight is Valentines day, so... i may wait until tomorrow.