I haven't looked for one @ WallyMart = I dunno if they carry them.
I bought My Ruffle Foot at JoAnn's, a long time ago. (Approx. $15-18 ?)
I've seen them at Hancock & Field's Fabrics, too.
IF the ones you find at those stores don't work for your older machine, then I'd suggest checking the Regular Sewing Centers.
They'll probably either have one instock, or can GET one.
OR just go there first. I'm not sure how much their mark-up is, it might not be much.

IT'S TOTALLY WORTH THE TRIP TO THE STORE !!!
After you get used to using it, you won't believe how you ever lived without it

!
Here's pictures of mine, it's original packaging, and my foot I compared it to.

As you can see my machine's not "new", from the 60's or 70's (dunno exact year made).