I'm not speaking from experience with any of the FWBs, but RWS Hobby or Geco would be a good starting point.

Bisley Practice would be another widely available pellet to try.

There is also a new H&N pellet, just called Econ I think, which might be worth trying?

RWS and H&N also make much more expensive match-grade flatheads as well, of course.

Regards,

Iain