Most well known third party venders are OK to deal with… however, you need to be careful of deals that sound to good to be true from a lesser know vender.
I bought mine from Amazon.com… that being said, you need to be aware that Adobe will be releasing CS5 and you might want to make sure you are in the window of free upgrade “time period”. the first version I bought was only 6 days outside of this window and I had to pay for a full upgrade (instead of getting it free).
Also, be aware that CS5 is 64 bit only… as a result… I would hold out on getting any third pary plug-ins since they will be useless with a 64bit ONLY software. 32bit plug-ins will not work with a 64bit (only) software. With that limitation, CS5 will make some expensive collection of third-party plug-ins obsolete. I hope Adobe decides to make 32bit plug-ins compatible with the 64bit system… but, I doubt it.