You might want to look at this thread, which I started a few months ago. I was looking for a good straightedge to flatten my table saw top:
While shopping, I learned that some rulers have labels on them that say "straightedge", others not.
I found an Empire 48 inch straightedge / ruler (model 4004IM) that met my needs. I paid around $10 for it in my local hardware store, but it was untagged, the last one on the shelf, and not in their inventory, so they made up a price and gave me a deal.
Amazon sells a Starrett 48 inch straightedge ruler for $14. I don't own one, but a Starrett brand straightedge better be straight, right? A concern would be protection during shipment.
All the straightedges and levels I looked at in the stores were wrapped in shrinkwrap, so you could not test the edges for straightness. I finally decided to buy one anyway and take my chances. It worked out okay for me.
One problem with using straightedge rulers is that they don't stand on their own. I use blocks of scrapwood to hold it up when I need a third hand. A better solution might be a self-standing level. My old long level has too much accumulated cement and other gunk that prevent its use as a straightedge, which is why I bought the new one.
I hope this helps.