If you are especially serious about simulating the behavior of a product's circuit you may have several models for a single part. One will be based on "nominal" datasheet values of course, but you may also have models for best-case and worst-case examples of the part. Or a model which combines best- and worst-case values for various parameters to create a part representing a "corner-case" for a particular design. Or models representing behavior at high, or low, temperatures. Or a model optimized for accuracy under certain operating conditions (e.g., low current, high voltage, etc). Or models based on experience with the particular strengths, weaknesses, and quirks of parts from a particular manufacturer. Or a model that strips a device's behavior down to the bare essentials, to reduce simulation time - or a model that incorporates every parasitic element and blemish that characterize a particular device.
While some of the SPICE-based simulators have impressive features or unique capabilities the underlying models themselves are quite portable from one vendor's program to another. Occasionally you may do a little text editing to circumvent a particular vendor's "extension" to the SPICE syntax but this is fairly rare.
The Component's manufacturer is probably the best source of SPICE models, though even some of these are of dubious quality. After the manufacturer, I look at the LTSpice User Group on Yahoo ( https://groups.yahoo.com/neo/groups/LTspice/info ), and then to specialized discussion forums like www.diyaudio.com .