SDP/SI sells 0.1475 in/3.75 mm pitch chain that seems to be roughly similar in design to ANSI B29.1 #25 chain, but smaller. (Search their catalogue for part numbers beginning with A 6Y 7-M or A 6Y 7MM.) Sprockets are available as well.
They also sell snap-together plastic chains in 0.1227 in/3.117 mm pitch (A 6M 7-12 or A 6M 7M12), and matching sprockets.
And of course, if you want #25 or #35 chain, they have that too, in steel, stainless steel, nylatron, acetal, or polypropylene, with metal or plastic sprockets to match.
If you're not so interested in power transmission, they also have various kinds of ladder chain.