Hardwood lumber suppliers are often a good source for plywood veneer sheets. Home improvement stores such as Lowes and Home Depot have limited supplies. Small quantities can sometimes be found at surplus stores such as Habitat for Humanity or building supply salvage yards.
Availability of plywood veneer sheets depends on the type of veneer desired. Speciality hardwood lumber suppliers are more likely to have walnut, hickory, and other fine veneers. Home improvement and building supply stores usually have oak or birch veneer plywood sheets. Sometimes sheets with small imperfections are sold at a discount at lumber yards. Formwood is a leading manufacturer of veneer plywood and sells to the general public.
Unless you need a large quantity of plywood, it is usually more cost effective to purchase it locally if possible. Hardwood flooring and cabinet dealers sometimes stock veneer plywood or can recommend a place to purchase it. Veneer plywood comes in several thicknesses with the thicker varieties costing more.
Craigslist.org can have listings of plywood that is left over from building projects. eBay.com has plywood for sale and can be a good option for those that just need a small quantity for hobby projects. Woodcraft offers variety packs of veneer plywood for the hobby woodworker.