There's normally 2 or 3 trailing logs on Ebay. Don't know what they go for as never bid, but the starting prices are normally a couple of quid (unless they are being sold as collectors items). I've seen a few complete with box and spare trailing impeller starting at £10. That's probably your best bet.
If you're really hard up you can make one: long piece of string, tie some knots in it and a log at the end and chuck it out the back every 15 minutes, say, to count the knots.
Not sure how long it needs to be and how long you should count the knots for, or how far apart they should be. You'll have to do the maths.