You'll find several at mac most.com. I've seen only a few of these, but those were pretty clear and reasonably paced.
Check their home page for other subjects.
You can download the 'Numbers'09 User Guide', also 'Formula and Functions Help' from the Numbers Help menu. They are a manuals, not videos, but are useful resources.
Also, in Numbers you can click on the Function button in the Toolbar. Choose 'Show Function Browser'. I find that helpful.