I bought a table saw for $20 without a blade or a blade nut. Actual replacement blade nuts are special order items for most hardware stores. Turns out the thread is a 5/8″-18 UNF nut. Some hardware stores carry these, but it might be easier to find them as a lawn mower blade nut. Mine was less than $5 with Prime shipping: http://amzn.to/2b4hqrC
Some table saws have a left hand nut. You can tell because the motor would be on the right side of the saw.