For dark rums, try Appletons. From Jamaica. They have run of the mill stuff and they have some specialty aged stuff in the 12 and 18 yr range, but as mentioned, you pay for age.
I use Appleton V/X as a mixer with diet coke...aaah, what a beautiful thing. That rum is so sweet you can't even taste it. Dangerous i tell ya...they'll sneak up on you, and quick!