Sound effects are an integral part of a game, but finding the right one for a particular event can be tricky. Here are two of the best ones I know of and which I use frequently (not just for RPG Maker, but other projects as well). Best of all they’re free! (Click on the title to open the corresponding website. They open in new tabs/windows.)
This site has quite an impressive collection of sound effects. It also has some amazing music tracks. You'll need to register first before you can download anything, but it's worth it.
This is the definitive site for sounds in my opinion. As its name suggests you can find all kinds of sounds there. If there's a particular sound you're looking for, you can almost certainly find it here. Formats are mostly in MP3 and WAV.
This is another definitive site, with a plethora of user-uploaded sounds. You need to create an account to download anything, and specific sounds are rather more difficult to find, but it's a worthwhile site nevertheless. Formats vary, including FLAC and AIF, but most are WAV.
This site has a blend of high quality, unique sound effects, which have been "exclusively produced". Although many of the sound effects have a distinct sci-fi feel to them, under the "Fantasy and Magic" section you might find some sound effects you can use for skills.
This site boasts 10,000+ sounds! All sounds a free to download in MP3 format. What I like most about this site is that you can preview and download the sound effects without leaving the page too often.