The RTP - "run time package" - is the collection of sprites and other resources that come with RPG Maker VX. They are drawn in a top-down perspective, a la every Final Fantasy game before Cloud Strife came into the picture.
Isometric sprites are drawn from a different perspective than the RTP sprites. Of course, there are plenty of non-RTP sprites that are not isometric, but that's not really important.
In general, you will want to use top-down sprites at this point
because you would have to find a script (or find somebody who could write a script) that would allow you to use isometric sprites and tilesets (er, maps) in your game, a hassle that would not really do much to improve the graphics of your game in the first place.
However, since the RTP is naturally overused (and given your position as an artist), I'd advise you to edit RTP sprites (recolor/put someone's head over someone else's body) or make your own. I know that there are plenty of tutorials on RRR and in other places concerning spriting, so those
should help you if you decide to do some artwork.
If you can't be bothered to do that, then there are plenty of free resources available online for sprites, windowskins, etc.