SEO Issue!

Hello guys,
We have a web site for grocery items sales.
We have thousands of products there and the system is very complicated as it connects multiple store inventory to online sales inventory.
All of them are centralized.
Now we are ranking well and getting traffic for some low competition keywords. (Product names) This is a decent start.
Traffic is like 4000-5000 a month only. But the product searches are very low, and we need to rank for more generic keywords like "buy ginger paste online" or "buy wheat flour online".
Now my problem is since the system is so complicated the client is not very open to create categories for my keywords which I then promote.
Their developer is equally complicating things and charging exorbitant amounts for creating a TAG page when we can add keywords as tags on products and then a tag page with some description and tagged products would be the landing page.
Something what we have in word press. (I hope this post is still not too complicated now)

So what I am asking is does any one know of a SEO friendly TAG plugin (for Magento) that we can use to create targeted landing pages.
It should support a description (text) and a flat url. That is all we seek. And yes a FREE version or a off the shelf cheaper plugin..