Right.
Seatpost - From a local bike shop. They are one of the most important parts of a bike so you'd be hard pushed not to find one there.
Crossbar - Either nick a trolley and unscrew the bar and cut it down. Or if you're a pussy and you don't wanna get told off by some 90 year old woman, then find out the diameter you need for a crossbar, and order some steel/chromoly metal tubing off the internet.
Or you could always order a crossbar from somewhere like ScooterZone.