I just remembered I have 5-6 plastic shelving units in my basement. From about 16"x16" to 2'x4' heavy duty ones. Gonna measure the height of each and try to use parts of those in my topper (until I can build something).
You might look around locally for used and 'throw-away' wood pallets. You don't want the treated, reusable pallets, often called 'grocery' pallets, typically 40x48 inch and often painted or stamped with a blue or orange color or a brand name. Again, NOT those. You also do NOT want pallets that came from shipping bags of fertilizer. These will pick up the chemical smell and will be un-usable.
Look for the 'orphans'...odd sizes and shapes, busted, rough texture, thin wood, etc. And again, look for natural wood, NOT stained or coated or treated or discolored.
You can find these in heavy retail and industrial areas, or sometimes even pallet recycling facilities like Pro-Pak are in your neighborhood. They often put these 'throw away' pallets in a 'free pile' because they are not worth recycling. Another source might be the local power company. They often store power components like transformers outdoors and might have a few extras they will give away. Another source is 'powersports' dealerships. Motorcycles, ATVs and snowmobiles are all shipped to the dealership on wood pallets and shipping crates. Lots of usable wood there! Just drop by and ask. You never know!
When you ask the guy out back, or the person driving a forklift around, use the term 'throw away pallets' and they can often point at the pile of old wood pallets they need to get rid of anyway.
You can often find enough 'good wood' to use for a simple project like the legs of a bed, or a simple support frame for shelving, outdoor tables, etc. If you don't already have a pry bar/nail puller and a claw hammer those will come in handy when it comes to pulling the pallet apart.
Lastly, don't count on perfection, and you wont be disappointed!