What exactly are you looking for? I know a bit about converting from RPG Maker 2000 and 2003 to RPG Maker VX / VX Ace (and not the foggiest about RPG Maker XP), so basically everything I say is for doing that. Also note that I don't know of any places that host converted content; I mostly just have tools/techniques.
For Facesets, you should just be able to resize them to 2x using GIMP or something similar and the results should be passable. If you use FaceMaker (which was fairly popular in ye olden dayes), you should be able to save them at VX/Ace's default resolution of 96x96.
For Charsets, try searching for 'RPG Maker CharConverter'; If I remember correctly, this program will automatically rearrange charsets from earlier makers to match the format of VX/Ace. It's rather poorly translated Japanese, and if memory serves you need to select one of the options to make it resize (to 2x) or the characters are ludicrously small.
For Tilesets, you are basically SOL. I've seen lots of requests to convert them but no actual conversions. It CAN be done, and it's a relatively simple (but incredibly tedious) process of copy/pasting and resizing (again, to 2x). If you're interested in going through the hair-tearing experience for yourself, I might be able to raid my other computer for some templates from when I tried to do this. (I succeeded in converting a single Mac & Blue tileset; I could upload that too, I guess).
Panoramas/Background would need to be resized for the newer, smaller resolution (which is something odd that escapes me; you'd have to check the documentation), or you could find a Script that allows you to resize the screen to the older makers 640x480 (which changes the size-in-tiles of the map to the original 20x15 rather than the newer I-think-like-17x13?) I know either
Yanfly's basic engine or his battle engine has that as an option.
Enemy battlers would probably look fine, size-wise, at a 2x resize.
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Some people don't like the 2x resize because it makes everything so obviously low-res; YMMV (mine does). 'Course you don't have much choice if you want to reuse the old stuff.
As for where to find content, Charas is the only English site I can think of offhand. I usually do some heavy googling if I'm looking for stuff (or use the old Charas offline generator). As far as tilesets go, I've always liked
Mac and Blue and
Theodore's stuff. You might also look around
RPGMakerVXAce.net and the
official forums; you might find a few topics talking about older stuff conversions.