Hilary- Congratulations on landing a nice big account.
Sounds like your casters just don't want to do it. Fine silver is
not that much more than sterling. A little, but certainly not more
than three times the cost of sterling. It's easy to just buy common
fine silver rounds from most any bullion broker or refiner. I get
mine from AAA Precious Metals here in Portland.
Fine silver needs no special casting equipment or techniques. I've
done it tons of times. It should be the same cost per casting. You
may want to offer to supply the metal and have them just charge you
the casting/flask fee.
If they balk at that, it's time to find another caster. There are
plenty out there looking for business.
Have fun and make lots of jewelry.