I've got Joomla 1.5 and Virtuemart 2.0 and I need to know if there is any way to add tags for products because I can't find any component for this purpose.
You can find extension in the Joomla Extensions Directory, infact they have there own category — Virtuemart Extensions.
As you're using Virtuemart 2 plugins should work on your VM2/Joomla 1.5 setup.
If you need to look for older extensions you can try the Joomla! 1.5 Extensions Archive which also has it's own Virtuemart section.