I use tegaderm bioclusive dressings ( it's the stuff they use to cover IV sites in the hospital). You can buy it at any medical supply house or some pharmacies carry them as well. I buy mine off ebay in large lots of 50 or 100 covers at a time, to save a lot of money. I use the 10 cm x 12 cm ( 4 inches x 4 3/4 inches) for the 100 mcg patches and the smaller ones for the 25 mcg patches. The 4 inch ones are 1626W is the id number for the ones by 3 M that I use.
I know that nexcare also makes one , those you can usually find in any pharmacy in the dressings section, but they can get expensive, something like 12- 14.00 for 5 of them.....so I use ebay as I said instead, much cheaper.
There is also something called opsite that you can buy in medical supply places but it can be a bit awkward to use. You can cut to the sizes that you need, but you have to have two hands available to remove the backing and put it in place and then peel the backing off.