Looking for a long lasting, FLAT messenger bag.

Check ebags.com.
I buy many things there.

I dont have a specific recommendation, but when I was looking for one what stood out to me was how much some of them weighed empty. Forget about adding papers and books. For men it might matter less, but for me if I had to carry a very heavy bag for hours it would be bad news.

Try Levenger
A bit pricey, but you will find long lasting and quality in the products.
Watch for sales and clearance items.

There are several options at Overstock.

These are great. Old Navy also used to sell a good one, but I can’t find it online so they may have stopped making it.

Timbuk2 make good messenger bags.

Have you checked out Tumi?
Edit: (I just realized you didn’t want expensive. )

Funny thing. . .I’m trying to get my hands OFF one. Unfortunately though, I’m not sure it’s the width you’re looking for. It’s pretty wide, but still lightweight.I can still send you a pic later if you’re interested. It’s a Swiss brand messenger bag, so it’s really durable. (The same people that make the Swiss Army Knife). I ending up not liking it because it didn’t go well with my style.

When I worked in the bicycle industry and was much more of a ‘bike guy’ I used Timbuk2 messenger bags, as @Crumpet above mentioned.

They’re pretty much the standard when it comes to messenger bags. When I lived in NYC all the bike messengers used them.

They come in different sizes and shapes, but they’re pricey. Very much worth it though, if you’re looking for a high quality bag.

Timberland. Here ya go.