If you can't get a suitable generic one from ASAP or similar, Marlexmarine do Stuart engine spares, could be worth a try.
IIRC, the original is made from perforated brass sheet. Ebay has many offcuts of thin perforated stainless steel, various hole sizes, at cheap prices. You might consider making a suitable tube out of this.
The new ones were plastic with pretty pink ends.
I have recently seen them (or what I thought looked very like them) in a Chandlers. As I have only visited this place recently: http://www.mountbattenboathouse.co.uk/
- you could give them a ring.
Thanks for the leads. My old one is plastic with pretty pink ends. It's very tight, and difficult to remove without damaging it. I think the metal gauze may be be more durable.
Pete
For the 20 years I've had this boat the strainer has been a rolled up piece of plastic mesh, and it works very well: pull it out, unroll, knock the muck off, reinstall and carry on.