You can also use PUL (polyurethane laminate, IIRC), which is found in the utility fabrics at JAF. It comes in several colors or prints and is generally thinner than vinyl and may be a little easier to work with. It's used in cloth diaper making, and I've used it to make waterproof covers for my dog beds. Because it is made for hospital and diaper use, it's easy to wash, which would be good for a walker bag. The stuff is generally $10/yd (for solid colors), but I always use a 40-50% off coupon.