How can I get The Irregular at Magic High School's English-translated novel?
Yen Press has the license for the light novels. As of this moment, there are exactly zero volumes translated, but volume 1 is scheduled for April, and volume 2 is scheduled for August. I advise you to check their New Releases page for future updates about when they release additional volumes.

There is not really an "official" site for this sort of thing in general, as far as I know. Your best bet is probably to identify the licensor for whichever series you're interested in (check ANN, or just Google), and then look at the licensor's website.

(Yes, this is significantly behind Japan, which is actually at volume 19 as of March 2016. This is perfectly typical for English-language light novel translations. Compared to manga, there is relatively low demand for light novels, and they are relatively more labor-intensive to translate, so, honestly, you're lucky if you can even get a licensor to translate an entire series rather than stopping half-way through.)