high capacity

Ebay, but as they are not genuine nokia ones, most people feel they are inferiour. I wouldnt either, but up to you.

In most cases, you need a bigger battery cover.

Yeah "a high capacity BL-5F" is a bit of an oxymoron actually as the BL-5F is the official Nokia battery and only available in the one flavour.
Anything higher capacity is not offical and thus, not a BL-5F. Like bartmanekul, I wouldn't touch an unofficial battery with a bargepole, they regularly leak, expand and even explode and Nokia will not fix damage caused by one.