If you have the full version of Kontakt (e.g., not Kontakt Player), the Corebass Pear sample set from Orange Tree Samples is good. So is the upright bass in Native Instruments's Cuba sample set. But you'll need Kontakt for either of these, and if you don't have it, I'm of no help.
There's also a "jazz upright" in Kontakt that uses aftertouch to regulate the pitch of the note, letting you get a lot more realistic with the sound. However, it's very difficult to use in practice--getting the note anywhere close to on-pitch is difficult. It's probably meant for someone who wants to do a lot of editing to the MIDI scroll.
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