Purchase 2 x 15 litre water bottles from your local supermarket and using 2 ochy straps, secure them to the middle sides of your marquee as shown below.
Suggestion 2
Wrap around leg weights, available for puchase here
Suggestion 3
10kg black sandbags on eBay for approx. $15 each plus postage.
(Boutique markets will order these items in bulk on your behalf, to save on postage if there is enough interest, but an administration fee will apply.)
Suggestion 4
Make your own - download the pattern for the image to the right.