Just normal stainless, ideally butted.
Sapim spokes from the cycle clinic.
I bought a box of decent (DT Swiss Revolution) spokes for a wheel build from one of the German mail order giants for about half what they cost here.
www.cyclebasket.com have an endearingly basic website and cheap ACI spokes.
https://www.cyclebasket.com/m9b0s454p1482/ACI_Alpina_Stainless_Steel_DB_Spokes_Silver_Box_Of_144_
Rose bikes is normally a good source.
I think my last wheel build, I sourced spokes from starbikes.
+1 for Rose bikes, and you can buy the quantity you need instead of a box full.
Bike24 are usually great for spokes and you can buy the quantity you want not just a box
Ta. Its at least a box I want but was wincing at the postage from Germany.
Sdeals
www.ryanbuildswheels.co.uk every time.
ACI double-butted from Sdeals or Cyclebasket - perfectly good and reliable for regular builds, never had one break on me. No need for anything fancy unless going for lower-spoke count or ultra-light builds and I'll buy from Germany - UK prices for DT or Sapim are a joke.
Spa Cycles for sapim but don't expect rapid delivery 😉
Halo spokes on eBay are pretty inexpensive, although they're plain gauge.
Ryanbuildswheels ebay shop. very good.