Usually human cells accumulate random genetic mutations over time that get passed down to other cells that they divide into. Each cancer is different but there are some mutations that are "the usual suspects" to look for. For instance mutations in the p53 promoter region will disrupt the cells ability to self destruct when it detects mutations. Obviously that contributes to the likelihood that it will become cancerous. This way of doing things is fairly similar to the way in which cancerous mutations would arise naturally in humans, but it is much less random.

Oh yeah Jackson labs is huge for providing mice tailored to your specifications. This sort of service is very useful because it keeps different labs results more consistent. So you can refer to the specific lineage of mice you are using and everyone in the scientific community knows what you are talking about and where to get some. But it is also very common to create your own mice in house. Microinjections can be tricky tho.