I recently read the snake amongst the flowers. I loved the regulus/lily ship so if you have anymore, I would love to know. I would write one myself, but I only do well writing canon ships.
I think your search would be much better suited over in the Fiction Junction (yupp, that's a link!)
You could post something in the 'What I'd Like to See" forum and you might get a few stories pop up. I've checked through the Recommened Romance fics and theres no Regulus/Lily stories there, but there is a Regulus/OC, here, if you'd like to try that one out. There's also quite a few fics on Sirius/Lily if you want to give that Ship a try. There might also be some more things in the other Recommened fics, perhaps try Marauder Era.
And who said you can't write Regulus/Lily??? There are plenty of posts about Regulus and Lily both to help you understand their characters more. I'm sure you'd be able to write a wonderful fic with some help. You'll never know until you try!