Which journals publish 1-page papers

There are plenty of examples listed here.

Generally, journals rarely seem to have an explicit lower bound on the lengths of the papers. If they do, they will explicitly tell you so, and in those cases, the same society often publishes a journal for shorter papers.

If you proved an important result in a readable way on one page, almost no decent journal would ask you to add a bit of waffle to fit their criteria. So the question you should be asking is "which journals publish results of ---fill in--- importance/generality or on the topic of ---fill in---". Having said that, there are a few journals who particularly specialise on short papers, i.e. have an upper bound on length rather than a lower one. In addition to what has been suggested, you can have a look at the Bulletin of the LMS (the LMS also issues two other journals for longer papers). Also, the Bulletin of the Australian Mathematical Society promises short turnover times, which suggests that they like short papers. If you think that your paper is first class, then you can also consider the Annals of Mathematics, since they have a policy of encouraging short papers (by which they mean under 20 pages).

But as I say, I genuinly believe that no journal is going to turn down a paper because it is too short, unless this is their explicitly stated policy. So just choose a journal that you think is interested in your result.

On the other hand, unless you think that the brevity is a selling point of your paper, why don't you add a few examples to illustrate the interest or the usefulness of your result?

The American Mathematical Monthly publishes several short "Notes" in each issue, and I'm sure I've seen several that were only about a page. As you would expect (being published by the MAA), many of these are mainly of pedagogical interest, but there are also some very interesting and nontrivial ones.