Hi and Welcome!
Are you missing the springs and ends caps? Is the tube bent? Many times the tube can get bent from heavy rains with the awning not pitched enough, but the tube is replaceable and less cost then the entire assembly. We can show how this is done if needed.
Reason for asking, if you have the existing springs and end caps, you can buy just the tube, reuse your old springs, end caps and replacement fabric if you need new fabric. And for the fabric, there are several choices.
As far as the awning tube, in 2019 I bought 2 tubes from Dyers RV on line. They drop shipped direct from Dometic in Elkhart to my house here in Ohio. I was after the 19' awning, and you asked about a 21'
See here for the 21 ft tube from Dyers. Freight only cost $10 the last I bought one. They roll the cost of freight into the tube price.
https://www.dyersonline.com/dometic-21-aluminum-roller-tube.html
RVpartscountry also sells them, but they charge full freight, point is shop the total cost to you price
https://www.rvpartscountry.com/RV-Dometic-21-Ft-Roller-Tube.html
If you actually are missing the entire tube, springs and end caps, and fabric, you have to find the assembly part number. With the Covid and the price increases, many RV parts are really a lot more then they where in 2019.
Hope this helps
John
PS. Check the part numbers before ordering. Call the place you are going to get it from and give them the serial numbers off you old tube. Assuming you have the old tube with the sticker on it.