On a hunch last night, I went to Walmart on my way home. I didn’t find spanx, specifically, but I did find some “light control” underwear in the shapewear section. Because I’m working out for weight loss reasons and get measured regularly, I could figure out the “correct” size for me based on their sizing chart and my hip measurement and got a couple of correctly-sized ones, and one that’s one size smaller than it said I should get. I tried the smaller size last night with a cloth menstrual pad, and they fit fine, and worked even better than just a cloth pad alone. The underwear was also really inexpensive for what it is. Just under $10 for 3 pairs.
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