I just made some out of 10mm (o.k. that's 3/8 inch) soft copper pipe
that's used by plumbers for central heating systems. You cut a 10mm
length, place it one end down on a hardwood block and gently bell out
the top with a hammer and the rounded head of a coachbolt. Then you
turn it over and do the same with the other end, if necessary after
passing it through the hole in your fabric. *However* there's a trick
which makes it easier. Fit a small stud (e.g. a piece of dowel) of
dia. = to i.d. of tube, into surface of hardwood block, projecting,
say, 3mm up from block face, and locate your thimble blank over this
stud when shaping.

Buying them is probably easier but this is more fun.