I got mine at REI Outfitters. The umbrella is built to clamp onto a chair but works perfectly for clamping to a stand. $5.99 when I got it.
Standard umbrella disclaimer. If you use one make sure to bring tent stakes and bungie cords or your stand will go over at the first big gust.
I have an umbrella with wind vents in it so when it gets a good gust, the wind travels thru the umbrella slightly instead of getting caught up in the umbrella and knocking it over
I got it at Wal-Mart about 5-6 years ago at least tho for around 10-15 dollars! Well worth the little extra $$ compared to a regular umbrella
I agree tho, I ALWAYS have tent stakes and lots of duct tape so my stand is VERY STURDY and the wind doesn't knock it over. The vented umbrella helps alot too tho!