Hi, I have a player who needs a deity that is lawful neutral.... Or lawful good, ideas?

Your question is a bit ambiguous—are you looking to homebrew a god or are you looking to use an existing pantheon?  If the second option, what setting are using, as that will dictate which gods are available.

In either case, can you provide more information about your player?  They likely would worship a good related to their character, which is another useful way to begin your search—you can either homebrew something your character would worship or use their character to narrow down which god to pick.  

Answering based on the Critical Role pantheon (which is a subset of Greyhawk):

Raven Queen (goddess of death and fate) is a great LN goddess to follow.  Kill all undead and necromancers that you see, and you're following her commandments pretty well

Erathis (goddess of civilization) is another great LN goddess.  Defend towns/cities, do quests, etc, and you're following her commandments

For LG you could use Moradim (great if you're a Dwarf) or Bahamut (great if you're Dragonborn or just like dragons).

There's the old Greyhawk standby, St Cuthbert.  Why think about complex moral issues when you could hit people with a club instead?

Some forgotten realms ideas: Helm and Kelemvor are textbook LN while Torm and Tyr are textbook LG. What class/race is your player?

Back before wotc gutted/sanitized lore in D&D, there used to be a number of books that created entire pantheons of deities. One of the books that would help you a lot is an old old book, long out of print, but virtual copies can be purchased at at sites like RPGDT. That book would be AD&D's Deities and Demi-Gods.

Appendix B: Gods of the Multiverse in the (5th edition) Player's Handbook has a listing of many deities and their alignments. You and your player did buy it, right? 

You don't even necessarily need to buy; the Basic Rules gives the initial FR deity list as well as alignments for deities from historical pantheons.

Just pointing out a free resource for the question exists on this site.

LN Faerun: Azuth, Helm, Kelemvor, Savras. Greyhawk: Ulaa, Wee Jas. Wildemount: Raven Queen, Erathis.

LG Faerun: Ilmater, Torm, Tyr, Bahamut, Garl Glittergold, Moradin, Yondalla. Greyhawk: Heironeous, Rao. Dragonlance: Paladine, Kiri-Jolith, Majere, Mishakal, Solinari

Plenty of others depending on the setting. 

Unless there was a retcon, St Cuthbert and Pholtus are both Lawful Neutral deities, not lawful good.

Quote from 6thLyranGuard >>

Unless there was a retcon, St Cuthbert and Pholtus are both Lawful Neutral deities, not lawful good.

 I just checked again and came up with LG for Pholtus and LN for St Cuthbert but Another source had that one as borderline LG/LN. I'll just edit both out of my post to remove the confusion.

I believe you, but that is quite a weird choice on the part of whoever wrote that since Pholtus was historically LN with LE tendencies.  He was called The Blinding Light because he was a sun god who's doctrine promoted following laws blindly and promoted human supremacy: half-elves and half-orcs were at best second-class citizens and he had no use for non-humans that were living on land and using resources that could have been used by humans instead, if you get my drift.

I'll PM you a handful of sources and you can take it up with them. :)