The fabric you can get at Joann's which frequently has sales. You can also purchase at efavormart.com. It appears to be chiffon and may need 6-10 yards depending on the look you want. You can always trim the excess and use elsewhere.
Daughter's florist used zipties to attach her floral pieces to her arch -- they held tight and were easy to cut after the ceremony so the flowers could be repurposed at the reception. (You can buy zipties at Home Depot or another hardware store.)
If you buy fabric to make your own drape, I'd probably try to do some prework on it and potentially sew the pleats into place for the two upper corners. "Draping" looks casual and effortless, but it actually takes some work for it to look right. You may want to see if your venue will let you come in advance to work on figuring out how you want to adjust the fabric on the actual arch. Good luck!
You can get that exact decor from the photo by going to Lings website. It’s a mix of silk and foam flowers I believe and costs $60-80 I think for two floral swags and arbor drapery. I also think they sell that brand on Amazon
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Oops! Didn’t read the whole thing thoroughly lol I thought this was just a Pinterest pic you sent for inspo. I bought mine on amazon but I’m not sure if they had that burgundy draping. I bought the burgundy and dusty rose one but they had some others as well. It came with rose draping but I might just use it as a table runner instead since we’re doing one long table for all 20 guests
Here's another one from the same one for just the flowers and then you could find burgundy drapery at joanns or something like that Link without drapery
Mine came super quick with 2 day shipping. I'll probably add in more faux florals or greenery on my own to fill it out even more and give it more fall colors. Hope that helps! You can't beat this price for arbor florals!!