Where exactly can I find the FAA's UAS pilot training course (ALC-451)?
Even when faasafety.gov is working without issue, the course can be difficult to find with the site's search feature. The Part 107 Small Unmanned Aircraft Systems (small UAS) course is presently available from this direct link: Part 107 Small UAS Course.

Note that you will have to be logged in to do the course. If you follow that link and are not logged in to faasafety.gov, log in and then go to that link again.

Note also that faasafety.gov currently seems to be experienceing occasional technical issues, either due to maintenance or traffic volume. The link may not work reliably at present.