The easiest is to go to just about any online outdoor retailer. Not that REI is any better than any other retailer, but here's a link to an MSR windscreen on REI's site. $15.00 + shipping is the going rate.

You can also check eBay. If one is being offered, sometimes you can pick it up for less than retail.

You can also MYOG (Make Your Own Gear). The lid to caterer's food containers is about the right gauge. Some people use aluminum flashing from the hardware store, but I find flashing to be too stiff.

If none of those options appeal to you, PM me, and I'll make you one. I make my own custom windscreens, and I have some sheet aluminum of the appropriate gauge.

HJ