I've just been on a little MYOG ordering binge after watching too many youtube videos.
https://dutchwaregear.com/Easy to navigate, fast standard shipping (I'm sure it was here in less than a week) and shipping was about $35 on a 1.1kg order. They wouldn't ship insulation like climashield but will ship a pillow kit which contains half a yard of climashield

I got some extra wide silpoly fabric which should be interesting to test.
https://www.extremtextil.de/en/Has a large range of good fabrics (including interesting bits like TPU coated fabrics for packrafts etc), shipping was about 8 Euros for a few metres of fabric alone.
https://ripstopbytheroll.com/Probably the most commonly recommended, but I haven't ordered with them.
https://www.diypackraft.com/Only place I found that sells inflation valves for a reasonable price, but obviously specific to packraft materials. $11 shipping on a $30 order.
Edit: aliexpress.com has an awesome range of miscellaneous buckles, tent parts, poles, pegs etc. So far most stuff I've ordered has been excellent quality and a fraction of the price elsewhere. I still don't know if I'd recommend it for mission critical stuff, but well worth a look. Sometimes it takes a lot of searching to find what you want.