Don't waste your time with an issued sling; they are garbage and not too many pers who deploy use them. Your best bet for a sling IMHO is something from avail from a Canadian merchant that soldiers would buy from;
http://www.oneshottactical.com/Merch...ry_Code=SL-BFG
http://www.cpgear.com/StoreBox/slings.htm
https://dstactical.com/index.php?cPath=161_195
Regarding the helmet strap, we have a surplus store here in Kingston that has bits and bobs, I could take a look for you and see if they have any in size (L). Please shoot me a PM to remind me; if I can find legit surplus I don't mind helping a collector out.