hey there i was wondering if anyone could help figure this out. Neil Young plays my my hey hey (the acoustic one) with his guitar tuned down a whole step. He uses a Bb harmonica accompanying it. If I wanted to play the song only tuned down 1/2 step could i still use a Bb harmonica or should i use a C harmonica?
You need a a harmonica that matches the key you are playing. Whether you tune down one or two or six steps it's a matter of what key you end up playing in.
Neil is a very basic harp player... Much in the Dylan mode. Just match the harp to the guitar.

Note... Finding harps in odd keys like B-flat may take some searching, but they are available.
Unless playing harp is something you do a lot play in the keys you already have. Stick to having just the major keys and learn to play a song in one of those keys. Cheap harps are really garbage and are hard to play (Jambone and budget Hohner's come to mind) and good harps are getting way to expensive to have one in every key just for one or two songs. A nice Hohner Marine Band is now $30.00 or more, a Hohner Special 20 will cost more than $40.00 and so will a standard Lee Oskar. I recommend just sticking to major keys and play your guitar in the nearest key to the harmonicas you already have. Get a capo for your guitar if necessary.