I've found the same problem.  The custom made ones fit far better but then
you'll pay a whole lot more for them.  Fabric is often thicker so they wear
better.

I've grown fond of real sheepskin covers for their warmth and comfort as
they are really thick maybe 3/4"-1" or so.  They come in more colors than
you can think of and can be fitted if your seats have the airbags in them
(they have some Velcro blow-out seam).  Drawback is they cost big dollars
(mine were $450 for seats, backs, and headrests) but they also last far
longer than the generic auto-parts store covers.

Advantage is they don't shed or very little.  Another is they don't have a
static buildup and leave clingy fabric frays all over your clothes like
phony acrylic and polyester covers.  Major drawback, other than cost, is
that they take some time to air out as they can smell like a wet dog on a
hot day.  Takes some Fabreeze treatments and brushing with a metal-tipped
dog hair brush to get rid of the "sheepy" smell.

Mine came from a seat cover place I found in the local phone book.  They
take some time to get (6 weeks for me) as the skins are from New Zealand and
they are sewn and dyed back east somewhere.

Good luck.

B~