Better check your strap width first. Odds are good that it’s 18mm, but it could also be 16mm, 20mm or larger. Get a metric ruler and find out.
Then hit up eBay or just google search for NATO straps. Those seem to be the straps of choice these days. I’ve heard that American Eagle has quite a few 18mm ones super cheap once in a while, and J.Crew often puts theirs on sale, too. All comes down to how much you want to spend.
The Brooks Brothers straps (I have a few) are actually 16mm. Fortunately, I have two watches with that width to use them. You might not be so lucky.