Can I still find hard to find NPC's that are ranked under my current rank 103?
I am always trying to find npcs that i havent seen for a while and wandered if i have passed a rank limit for when you can see them and still battle them?

If you've passed the rank limit you can't see them anymore. What NPCs were you thinking of in particular?

T.O. Cruiser
Blade Harvester
Sabre Gunship
Crimson Armory
Akkiren, Drannik Hub
Science Pod
Research Probe
Tech Cruiser
Tech Surveyor
Sha'din Corsair
Sha'din Infiltrator
Yahnosh, Sha'din Hacker
Stryll Hunter
Svax, Stryll Punisher
Sketh, Stryll Predator

Ah, I wasn't expecting that many. For the alertable ones, you can check the wiki to see their rank range. As for the others, most of them either have a higher rank equivalent (for example, the Stryll Hunter will be replaced with the Stryll Skirmisher) or are Matched NPCs (like the Blade Harvester), which means you'll always be able to see them.

I dont want to miss a opportunity getting stuff from npcs if they arent gonna show anymore

I'm fairly certain anything you could get from NPCs before you will still be able to get, or an upgraded version of it. Unless you get to 600+ I don't think you start losing things.

TO Cruisers stop showing up after a certain rank (forget what it is)
Blade Harvester and Sabre Gunship are matched NPC's so you will always see them regardless of level
Crimson Armory will eventually stop
Akkiren, Drannik Hub will be replaced by a new NPC Okkilor
Science Pod, Research Probe, Tech Cruiser and Tech Surveyor all eventually stop
Sha'din Corsair will be replaced with Sha'din Raptor
Inflitrators stop showing up *sigh*
Yahnosh, Sha'din Hacker will eventually be replaced by Sha'din AI Matrix, and then Hypergrids
Stryll Hunter will be replaced with Stryll Skirmisher and then evnetually Stryll Desolator
Svax, Stryll Punisher will be replaced by Xavox, Stryll Punisher
Sketh, Stryll Predator will eventually stop showing up (no replacement)

graxis is sketh's replacement

aye... the Stryll Hunt Seeker you get from Sketh is replaced by the Stryll Prey Tracker from Graxis

bad news: Graxis doesn't drop Stryll Pathogens
good news: Graxis drops Rescued Stryll Prisoners