Podiatry felt is a padding material that is self adhesive designed to be cut into different shapes and stuck on the foot. That means that podiatry felt can be useful to get pressure of bunions in the shoe.
Typically for bunions the felt is cut in the shape of a donut, with a hole in the middle and then stuck on the foot in such away that the padding keeps the shoe from pressing on the bunion. Alternatively the podiatry felt can be cut into the shape of a horseshoe and also adhered to the foot to get pressure off the bunion. Adhesive tape is often used to keep the podiatry felt in place and make it last longer.
Podiatry felt is generally used as a short term measure to get pressure off a bunion from the shoe when other methods can not be used. The big disadvantage of podiatry felt is that it has to be kept on the foot and kept dry (it will soak up the moisture if it becomes wet). This is going to make the method of offloading somewhat unhygienic and necessitate the felt to be replaced every few days which can be a bit inconvenient. It is still a good short term measure until a more medium to longer term measure can be put in place.
The felt can often be purchased online (eg here)