Any Way to Download Third Party Spelling Dictionary?

Scrivener does a good job at spell checking, but there are some false positives that make it less useful.

For example, these words are recognized as OK by OpenOffice, but flagged by Scrivener:

oversized
outta (as in “get outta here”)
waterbed
GPS
Metamucil
PBJ
OCD
Starbucks
RV

Is there any way to download a third-party spell checking dictionary?

Thanks,

Al