You can have custom implants made at any time (I had my first ones at 35, replaced them with updated ones at 48), but you can only buy the parts from the shops up to a certain QL (I think it's 125). After that, you have to get them as loot from mobs or missions.
The first thing you need is the empty implant - Get one around the level you want your implant to be. After that, you need to hunt for the nanoclusters you want - Get them with a QL at the lowest
one level than the implant you've bought (e.g. if you get a QL50 empty implant, buy a cluster QL49 or higher).
Once you've got them, try to find an Engineer - Some of them work on a tipping system or will work for free (although they obviously won't advertise it
After that - Surgery clinic, and pop them in
BTW several of the nanoclusters are part of the Shadowlands expansion - So unless you have SL, you might want to find alternatives for NCU memory, Nano Cost modifier, Offense/Defense modifiers, weapon range, energy damamge modifier, and skill lock modifier.