Sourcing ferrite cores for switchmode transformers is sometimes difficult (RS have a decent range, but they're a wee bit pricey). Fortunately, it's easy to "reclaim" cores and bobbins from old power supplies, especially computers.
The biggest problem is that the entire transformer is often dipped in an epoxy or thick varnish that holds everything together. The best way to dismantle the core is to first remove the tape holding the two core halves together, then place the entire transformer in an oven at around 180°C for 10 min, or as long as it takes to soften the glue/varnish sufficiently. It's then possible to gently pull the cores out of the bobbin. The temperature doesn't seem to affect the bobbin material much.
See also http://shaddack.twibright.com/projects/method_salvage_ferrite_cores/ for some more info on removing cores.
The cores break quite easily, and I've ruined a few even though I was careful. Also, the bobbins in old transformers seem to be rather brittle as well, and I've often broken them while trying to remove the wire.
Here's the results of one attempt - two nice ETD34 cores with intact bobbins (I broke both core and bobbin on the third transformer!):