20mm!! jeez that's a bit of a hike ain't it. I think I only needed 6/7mm when I fitted 166 brembos on my 156.
You'll need hub-centric spacers at that plus some new longer bolts, obviously, so you'll be hard pushed to find some off the shelf.
Halfords do carry them, but stock is hit and miss at best, patching across different stores. if you're lucky you'll find some that fit. You're more than likely going to have to order some.
Don't forget you're going to have to do the rears too, otherwise your car will have 40mm wider track at front vs rear and the handling will go to pot.
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso