well, if you are in flash gamedev industry for at least a year, you know that almost all flash games are not high-end games because of the short development time (weeks, sometime few months). Developers just can't afford more time for a single game because the main and almost the only monetization schemes currently available are ingame ads and sponsorship. So average budget for a game is less than $5000.
Fortunately it seems things are changing a little bit: some devs are starting to switch to the micro-transaction monetization scheme, and to do that they have to increase the quality of their games drastically.
Back to your question:
As far as I know, Mochimedia is the largest source of flash games (defacto they are the main provider of ingame ads), but you can search for good games at indie and flash game devs' forums.
Further reading:
http://www.indievision.org/?p=1510 (some premium-quality games are featured)
http://www.lostgarden.com/ (blog about flash games)