There are a few options for finding simulation models.
Generally, if you have a specific part in mind, you should check the manufacturer site first. There will usually be a section called "Support" or something like that and in this section of the site the manufacturer will provide SPICE models. I have developed a fairly large repository of components just by going to the major manufacturer websites and downloading their parts libraries.
If it is a more general part (such as your LM317) you can sometimes find parts that are similar enough that they can be substituted in a SPICE simulation.
Finally, if all else fails, you can make your own models. This probably won't help you much for a linear regulator, but when you have transistors, diodes or other SPICE primitives, you can get the necessary information from a datasheet a use a .MODEL command to create your own model. For more information on using the .MODEL command you can try this site.
After a bit of searching, I too was unable to find a model from LT. But TI has provided SPICE models for this part. Two versions can be found at this website and at this website. You can just copy and past the models into your netlist. Let me know if you need help with this part as well.
When it comes down to it though, as our friend Olin would remind you, it is important to realize that SPICE models are just that. They are models and models only, if you only need the behavior of the part (for example linear regulated output) then you easily create that effect on your own.
Here are a couple more resources I have found useful.
- For finding datasheets
- A huge directory of SPICE models
Best of luck in finding what you need!