Here are some ways:
1. ask bed stores for foam offcuts
2. find old matresses they are good
3. buy some foam from stores
Hope this helps =-)
build
The cheapest would be the stuff that people give to you. Old foam mattresses can be cut down to fill the pit, but you'll need hundreds of them. Plywood, OSB would do for the siding. Probably not particleboard or MDF though, not enough moisture resistance. You could try asking at a recycling centre or a demolition company if they can help you out.
One could go to websites such as eHow, Global-flat and YouTube to find tutorials about Flatland BMX riding. One could also visit local BMX clubs to find out more about Flatland BMX riding.
Ebay, craigslist, yard sales.
One could find the BMX Museum in Portland. The BMX Museum is considered an offbeat museum. This museum features former BMX racer Gary Sansom. One can also read more about the museum through their forums.
whatever was the cheapest at Canadian tire
Someone can find contact information for Winstanley's BMX store by going to the Winstanley BMX website, and click on contact us. At that page you will find all the contact information you need.
you can find a BMX bike in halfords
One can purchase BMX stickers online at websites such as DansComp, Zazzle, Lux BMX, and Supercross. One can also find them for discounted prices at Amazon or eBay.
You would be able to purchase a BMX frame from a retailer that sells BMX bikes. You could probably even find some used/old ones from craigslist or ebay.
Probably at a yardsale, or a local community recycling center. Otherwise craigslist or ebay.
If one is looking to find a BMX park list in the United States you can go to a site such as USABMX where they have a vast majority of the USA's BMX parks. They also have BMX parks that are in Canada as well as the latest BMX news that you need to catch up on.