I would also suggest to Valda this page (which tells you that the Prepositional case normally ends in -е or -и in the singular and -ах/-ях in the plural).

And this page (which explains that certain singular masculine nouns have a "special Locative case") ending in stressed -у/-ю, and gives a partial list of common nouns with this ending, such as сад ("garden") and лес ("forest").