1. The Internet - the single entity IMHO that 
made cel collecting explode. Most everything 
else is an extension of this.

2. Yahoo Japan - the swap meet of cels as 
opposed to eBay, the department store of 
cels.

3. Anime Beta forum - one of the tightest 
communities I've been a part of.

4. Anime cons - it's always nice to see 
something in person before you buy it.

5. Slug - a one stop virtual library of cels.