Properly made hollow, blind spliced loops will neither pull off nor come apart and can be made with any hollow line that the fly line will fit into - including 400 lb test hollow spectra. They are ideal for large fish. IMO there are no properly made after-market loops on the market that I know of.
For most freshwater species I'd just nail knot the leader butt. You could always add another nail knot of 8 lb. test below the leader butt nail knot for salmon if you've never caught one before and don't trust a single nail knot
