The very *best* hay is Oxbow Timothy Hay - I use this for my chinchillas - but it is rather expensive.
Alternatively some farms do decent hays - might be worth asking on a good rabbit forum such as Rabbits United. For example; quite a few people on there have recommended a farm shop near me that apparently does excellent quality hay.
The Hay Experts do a great range of hays and lots of people also love Dust Free Hay but I've never tried them. The Hay Experts also sell Cavy Cuisine - a high quality Guinea Pig pellet by Oxbow.