I have NO DOUBT many others have had that tiny bolt hold open spring going flying off into the ether after withdrawing the bolt carrier group from the receiver. Actually, the first spring I lost was the magazine disconnect but I could live without that one.
So I found a place online
www.smallparts.com that stocks a perfect replacement spring .12" OD x .012" wire diameter x .440" uncompressed length. Cost is .78 Cents for a pack of five music wire springs--I splurged and ordered two packs.
I also ordered a pack of 10 stainless 302 springs .12" OD x .012" wire size x .563" uncompressed length...a bit more expensive at $7.33 for 10 springs but they might come in handy.