I bought some from the chemist (wizard warehouse) You will need to measure your leg so that you get the correct size (probably best to just take a tape measure with you to the shop, as different brands have different size guides). These ones cost me about $50 i think.
My sister bought some from our hematologist (sp?), and they cost her about $140 or so blink and they didn't seem any different to the cheaper ones that i bought.
My ones were knee height which i liked, i put them on in the toilets just before we board and is easy to go the toilet etc whilst on the plane wink
We have a blood clotting factor that runs through our family so we've been told it's REALLY important to wear them, and also to avoid alcohol and walk around as much as possible during the flight (i usually sit in the aisle seat so it's easy for me to stretch and get up more often. Also drink heaps of water smile
Hope this helps!...Take care, and have a nice time away smile