You could try Trusting You by DreamsofSpike at Bloodshedverse. The summary is:
Buffy says she could never trust Spike...but how far is he willing to trust *her*? Rated NC-17
It does involve handcuffs and that big issue of trust. The beginning may be heavy going but does lighten gradually until the lovely spuffy ending.
Hope you enjoy it.
You could also try doing searches Bloodshedverse and Elysian Fields (both of which are filled with brilliant fics!) With EF if you go to their category tab it displays all sorts there (i.e baby, time travel, claim, AU, etc) but with Bloodshedverse you need to use the search tab and they have a drop down list of what type of fic you're searching for.
The other sites you could try are Fanfiction . net, Just Rewards, All About Spike, Nocturnal Light, Sinister Attraction and Fonts of Wisdom The Crypt. With the first three just search for the pairing Buffy/Spike or Spike/Buffy (some sites are weird with their pair ordering).
If I think of any more that fit this query I'll put them up.
Good luck
Sarah