Where can I find the Impact Factor for a given journal?

If your institution has a subscription to Journal Citation Reports (JCR), you can check it there. Try this URL:

https://jcr.clarivate.com

Apart from the ones already mentioned there are two excellent open access options.

Eigenfactor is quite authoritative, and depending on field its metrics may carry the same weight as the Reuters stuff. They also offer the ISI metric (impact factor)using the exact same formula. You can search by ISSN, journal names, discipline etc.

However when I make my own decision on an appropriate journal to target for publishing my articles, then I use SCImago Journal & Country Rank. They have a wide variety of metrics, which include upcoming ones like H-index, the traditional impact factor, and other tweaked versions of the same. One of most useful functions on Scimago is the ability to compare a set of journals using a graph format and any metric that you desire. This can be exceptionally helpful in identifying the prestigious dinosaurs and the new but cutting edge journals. Placing your article is half the game, so this is incredibly useful.

That said, this site does not appear to have the same level of authority. So, while it is good for one to get a good feel of the quality of a journal, it doesn't carry the same authority in a formal evaluation as Eigenfactor or ISI. Plus, if I understand correctly, their database goes back only up to 1996, though their coverage of even lesser known journals is comprehensive. This would make it difficult to judge the impact of an article published way back (but for older articles, citations are the standard measure anyway).

I also feel it is a shame to be paying to access impact factor statistics. Even if you cannot publish open access, at least the rent seeking involved in paying for the ISI rankings can be avoided by using the open access rankings.

There is a list at Science Gateway.