[Ninja]

If there was no hop on your gun, there likely isn't an aftermarket unit for it.  I assume you have one of the Prokiller models with the extended barrel and the fake silencer.  If you have a single stack or Para-Ord model, then you can get any similar slide and barrel for it.  The parts all interchange along model lines.  
Spring kits are stiffer so there is snappier recoil.  It's mainly if you have a metal slide and use green gas, so the blowback is still quick.  
A front site would probably have to come from WA themselves, so I would try emailing one of the bigger airsoft shops like WGC and asking for the replacement part. Check the manual that came with the gun to get the part number.

[Gladiator24_7]

I want to pimp out my WA Infinity, but I ran into some snags.  I have an integrated silencer on it, which is really just the outer barrel with a super long inner barrel running to the tip with no hop (for accuracy)  

I figured I'd just buy a new barrel and outer barrel, but i haven't been able to find a WA arms hop unit for the infinity.  Do they sell one?

Also, what exactly do the upgrade Guarder spring kits do?

And lastly, I wanna buy a new slide anyway, but I seem to have lost my front sight tip.  Does anybody sell these?

Might just be easier to sell this one and buy a new one...