Editing Warhammer 40,000/9th Edition Tactics/Necrons (section)

===Converting===
You can convert a box of Necron Warriors into Flayed Ones with little amount of work (the guide for how to do so can be found on google). Flayed Ones are probably the models you should most consider converting because of their high monetary cost.

There's a pretty good and common conversion that turns 1 box of Wraiths into a Tomb Stalker/Sentinel.

Pre-9th edition Scarabs being more or less 1 sided (bottom can be flat) are perfect targets for molding with oyumaru and casting with green stuff. Helps you to make actual swarms.

You can make a relatively nice-looking Gauss Pylon with a globe, the cannon from a Doomsday Ark/Ghost Ark kit, and some plastic spoons.

The Nighthaunt are a great source of bits and alternative models for many of your units. [https://www.games-workshop.com/en-US/Nighthaunt-Lady-Olynder-2018] would, with minimal changes, make a decent Transcendent C'Tan, while some of the more specialized Nighthaunt Infantry would make well on-theme Maynarhk, Novokh, or Nephrekh units, particularly Flayed Ones (a full squads for $84 before bitz vs $220) or Kit-bashed Immortals and Deathmarks.

Slyvaneth models can be used for C’tan and Necrons. Especially the Drycha Hamadreth model for C’tan and Spite-Revenants for Flayed Ones. 

Wargame Exclusive also sells some nice bodies for Immortals and Deathmarks[https://wargameexclusive.com/shop/necrocyborgs/necrocyborg-deathmarks-conversion-set-5u/].
One Box of Immortals comes with an extra set of arms for the sniper rifles, so it would be possible to build 5 Immortals and 5 Deathmarks from one box for a little less than what Games Workshop would charge for the two units.

For non-GW options, action figures/statues of [[/co/|Dr. Manhattan]] tend to work as good C'Tan.