My embroidery shop (that I "support") has fabrics for boat reupholstering, including a good, thick clear plastic.

I use upholstery plastic cover (that OUR grandmothers used) or sometimes I use a clear shower curtain ( 8 gauge) depending on the use.

I used a clear plastic from Spotlight, and it sews well. Have used it for bags for the beach too,

I made some coasters and used clear plastic as the last part when the satin edge was stitched. I don't know what gauge it was but firm but pliable. Stitched well. I got it from the fabric store in the section where they sell table plastic covers. But I have also seen it in the Hardware store.

I buy from spotlight but sometimes I use the vinyl plastic that sheets comes in, depending what I have available when I need it

I have used clear vinyl for many types of projects. And, it does depend on how heavy the other fabrics are that you are using for your project. Here are some links to small zip cases I made using lightweight calico cotton fabric with a vinyl window. This is a light weight of vinyl, that can be used for a table cover, and drapes well. In small sizes, it is perfect for the little window in this case, and in small treat bags. I have also used a lighter weight of vinyl from a (paper) sheet protector (that you can clip into a 3 hole notebook binder). These are very thin, but do work well in childrens treat bags. For outdoor projects, I have used heavy, thicker clear vinyl which is harder to stitch, but gives a more stable/durable result (for projects that will be exposed to the elements). Look on-line at fabric stores such as JoAnn Fabrics or the Fabric.com websites. You can compare the thickness/weights of the vinyl.

I use clear vinyl, too, Sue....I make sure that I space the stitches out around the vinyl so that it doesn't punch out, know what I mean?