I'm looking for any fics where Buffy and Riley are together while Buffy is with Spike.

BELONGING by: DREAMSOFSPIKE

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=16580


Summary: AU: Breaks from canon after Season Four...Five years after the Initiative disaster, the world is a very different place, where society in general knows about demons and vampires, and have found ways to dominate them. Buffy is trapped in an unhappy marriage to a man who doesn't seem to want her, trapped in a world that doesn't seem to need her anymore...can an old ally and enemy, sold into her home as a slave, teach her the meaning of freedom?

WHEN WE FIGHT by: FER1213

http://www.nocturnal-light.net/fanfiction/story/521/When_We_Fight

Summary: Starting in the summer between seasons 4 and 5, Riley's away and some interesting sparks are flying between Buffy and Spike. But when Riley returns, Buffy's problems are only beginning.

The Memories Series by Carriann is brilliant. Also set between seasons 4 and 5 --

It starts here:

http://www.sinister-attraction.net/stories/c/hategoodbyes.htm

Here are a few for you to check out:

Iced Heat by missus_grace
Summary: It's the summer after Season 4 and Buffy runs into Spike one hot evening.
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=21617

Just a Summer Romance by Slaymesoftly
Summary: The sequel to "Are We Friends?", this picks up immediately after Buffy and Spike's night together. Can they remain friends now? Will it turn into something more? How will the Scoobies and Angel respond if it does? What about Riley? All questions to be answered....
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=24993
The prequel to this is Are We Friends?
Summary: Buffy is alone for the summer, and she finds a surprising new friend and companion to help her stay amused. Feelings develop between Sunnydales two super blonds and life gets complicated...
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=24981

Cheating on Riley by tahmoe
Summary: What if Buffy gave into the dance with Spike in the episode Fool For Love?
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=4324

Could Be You by Abby
Summary: You know you wanna dance. Spike knew it, the fans knew it, and this time, Buffy knew it, too. Begins in the alley scene outside the Bronze during Fool for Love and moves swiftly off-canon.
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=35656
**This one is a WIP, but was recently updated last month. Just so you know!**

Also, if you do a general search for Fool For Love, there are a few others centered around this episode that fit your criteria.

I have written two fics myself where Buffy cheats on Riley with Spike one is a one shot called Feeling it:

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=30255&warning=AO-AdultsOnly

And the other is a WIP called Tricky propositions:

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=33147

Hope that helps.

Charms of the Clarion by Eurydice

Summary: When the Council approaches Buffy with an offer she can't refuse, she finds herself in another world, with Spike as her necessary partner. Set early Season 5. - Complete

It can be found at Elysian Fields and on Eurydice's website Love's Last Glimpse.