yeah octivia or pitch shifter pedals will do a harmony of 3rds, 5ths, 6ths, and octives usually. sometimes you'll find one that does 7ths or 4ths or 11ths... etc.

i cant help you with any good brands, the pitch shifter i have is built into the amp...

just go to a music website like musicians friend and type in "pitch shifter" into the search and you should find PLENTY.