I did see some for sale online (cant remeber where but I just googled python water change and picked UK only results). Price was about £65 though so i made my own. It is very easy to make, all you need is some hose, a T connector, a tap connector and a on/off valve. Cost me about £20 from B&Q.
How to make:
sorry about the drawing
In short you have a tap connector going to a short peiece of hose, this goes to a T connector (pink) which has a short bit of hose which you can aim at your drain with a shut off valve on the end. The other piece of hose connected to the T goes to the tank.
To drain water:
Turn the tap on gently to fill the hose with water. When the water starts to come out the other end put it under the water in the tank, turn the tap off and open the valve on the short bit over the drain. This will then drain the water out. NOTE: your drain needs to be lower then your tank or it wont work, also makre sure there are no loops in the hose as this can screw it up too.
To fill:
Just close the valve off and turn the tap on. Make sure you turn the filter off when you fill and put the dechlor in. Dont forget to turn the filter back on again!
Takes me about 5-10 minutes to do a 20% water change now
Not sure about your tap. I know you can get connectors which have like a clamp on that holds the hose in place.