You say the holes are a little offset? Do you mean they're beside the corners, rather than on them?
There are 2 generic mounting patterns, the fist is on the corners, the second is next to the corners. If yours is the second, you should still be able to find coolers (LOOK harder). They were available on several video cards as well, I have an MX440 PCI here with that style sink.
That's another thing, passive sinks have better fins than coolers, so screwing a fan to a passive sink gives you better cooling than a standard cooler provides.
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