Here are a few places our veterans have found theirs:

Big R or Harbor Freight, they carry the 'All Terrain' tricycles that are the base for almost all of the tricycles used in the race. The trikes have big knobby tires and vow to have a weight capacity of 500 lbs!  They are usually priced at about $50.  BUT- Harbor Freight frequently has them on sale AND if you have-or know someone who has- an Entertainment Book, they used to have a coupon for a % off.

North Division Bicycle Shop: Bank of America found their awesome stock trike here, It was spendy, but it probably the quickest stock model entered.  

Spok-n-Sport has tricycles, they are small but well made.

Wal-Mart, The General Store, Costco are also rumored to have trikes 

Check Craig's list, yard sales, E-bay, Amazon.com: Just keep in mind that putting an adult on a child's tricycle and doing the race oval puts some stress on the structure..get one in good shape or reinforce it!!

We are trying to gather some tricycles to loan, especially for the chldren's races.