I have never seen this device, but according to the user's manual on the manufacturer's website, it is just ordinary silicone rubber tubing. If you measure the size (inside and outside diameters) of the tube that came with the device, you will know what size you need. However, very few hardware stores sell silicone rubber tubing, as it is a specialty material, although some industrial (as opposed to homeowner type) stores do have it. It is available from some surgical supply stores(but do not confuse it with latex tubing, particularly if you have a latex allergy.) Smaller sizes are available at hobby stores, because it is used as fuel line in some models. A variety of sizes can be obtained in small quantities at only moderately inflated prices from Small Parts Inc. Their website is:
http://www.smallparts.com
From the main page, click on COMPONENTS. On the page that comes up, look under High Performance Tubing, then click on the link for Silicone Rubber Tubing. Then find the size you need in the table of stock numbers. You will have to buy a minimum 10 foot length, which will run you $25 to $30, according to the size. You must either call them to order the desired stock number, or click Shopping Cart and manually enter the number.
Another supplier, which has a variety of different formulations of the silicone tubing, is Cole Palmer,
http://www.colepalmer.com, and also McMaster-Carr,
http://www.mcmaster.com.