It seems that there are people out there with an 
unlimited bank account.  However, sometimes you 
can find great deals on ebay.  Once, I bought a 
great ssj4 Vegeta for only $35.  The next week, a 
SSJ4 Goku from the same sequence went for $350!!  
I find the DBZ web ring to be a good place to 
start.  Anime Taro is also good.  They currently 
have over 200 DB cels.  

I have a list of about 15 DBZ/GT cel dealers on 
my home pc, most of them update , if you are 
interested send me an email, and I will send you 
the links

Hope this helps

p.s.  You can always look at my sight ;)