'Truebones' do mocap and animation, offer a bunch of FBX budget/freebies for free membership.
https://truebones.gumroad.com/
Their sister site offers a ($$) utility that claims to convert MMD rigged-PMX and motion files to generic rigged FBX.
Excluding TDA-derived PMX models, which have strict 'NO PORT' T&Cs due '2nd Life' piracy, this may allow porting a zillion rigged PMX freebies to FBX for use in Poser.
Or it may not...
Poser is very fussy about its FBX rigging, preferring authentic 'Autodesk-SDK' flavour. 'Open Source' versions such as 'Noesis' or 'Blender' often import as multiple sub-rigs. A few sub-rigs are okay. Getting 10~~20 is manageable, as may include 'options' you can identify and delete. Some have 20~30~~50++ sub-rigs, choking Poser's hierarchy root. Even 'parenting' them to the first, 'master' sub-rig may leave Poser unstable...
Whatever: As Truebones does not seem to offer student / hobbyist discount, a trial version/period or even 'meaningful' FAQs, I'll need convincing.
I've sent an e-mail asking for clarification.