I would vote against the Renstore. I bought a similar kid's tabbard from them years ago, and was disappointed. It was just a rectangular piece of polyblend cloth (probably $4 in material) with a hole in the center. It looks okay with it belted but certainly not worth the $18 I spent on it + shipping (I think it was like $12!). And if it's a hot day, that poly material will drench you in sweat.
I guess I don't have so much a problem with the fact that it's a square (I sort of expected that). It was the cheap cloth they used.
I'd also bought a similar tabard on ebay and it was large enough to cover a kitchen dinette set. Once again. Not a happy camper with a purchase.
Eventually, I made a couple of 14th century surcoats using 15th century doublet patterns, and just extended the skirt length to mid calf. The pieces are very shaped, and they help define the body better, but still give a bit of a tubular look when worn. You're also not relying on a belt to give you shape. They aren't really any harder or easier then a rectangle to sew. It's just tweaking the dimensions.
If all you want is a square tabbard, I'm sure someone here can do it for you in a decent fabric, colors you want and to your size.
-Cian
This is one I made to fit over a plackard with faulds. I need to take it in at the waist and hips now that I have a globose breastplate:

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When wrongs are pressed because it is believed they will be borne, resistance becomes morality. -Thomas Jefferson