White blancoed sling could be RN - didn't they use white slings when on parade with white gaiters, web belts etc (KGV,KGVI funerals etc)....?
As to getting blanco today, the typical wartime colours Nos 61, 64, 97 and KG3 are reproduced by Pegasus Militaria here in the UK. It's as close to the original as you can get.
http://www.pegasusmilitaria.com/products/