Most any place with sewing capacity can make one up. In a lot of cases, larger companies won't.
Basically, only a felled seam may be new. Often, a local fabric shop will offer classes, including use of a machine, if needed. But, it is usually not hard to find one used, for a fairly good price, ie ~$30-50USD. You don't need anything except a straight stitch. No computerized countrols, no fancy zigzags, etc...A good driver or advancer mechanism is needed. Walking feet are often used and found at the sewing shop.
Material is either coated silnylon or cuben for light weight tarps, ~60" wide (5'.)
Seconds, cosmetic, are fine for most camping, bears don't care.
I have put together several. I am NOT a taylor!! Tarps are only roofs, if they stop sun and rain, it was good enough.