I really like the style of the Seagate Freeagent external drives, but I
don't need any more drives. Is there any source for the enclosures without
drives?
Very doubtful, since any spare external enclosure *overruns*
Seagate had, could still be fitted with drives and sold as
the full product so long as they make drives. Your best bet
is either buying the smallest capacity Freeagent you can
find so you have least money spent on the drive, finding a
use for the drive or selling it, or finding someone who
doesn't like the product and is willing to sell you just the
enclosure.
If you were talking about some product that was several
years old, out of production and at the end of the product
warranty period, it might be possible Seagate was
liquidating some parts into the grey market channels... but
even this seems unlikely for drive enclosures as I don't
recall seeing any drive-manufacturer branded enclosures from
past generations showing up anywhere at PC surplus 'sites...
and many people wouldn't even want them years later as they
might not support the capacity of the then contemporary
drives available.