I use Ptits Dessous inserts in my WN pockets (and sometimes boost with an Easy Peasy Hemp), I think a few people on here get on well with Greenkids inserts in them too.
Have the nappies always leaked or is it just a recent thing? If it's just recent then it might be that your nappies and inserts etc need stripping? If they have always leaked then it might be that your lo is a heavy wetter and needs more absorbency than you are using.
You are best posting this in the Pocket Nappies section as you'll probably get more replies, one of the mods will probably move it for you though