You have two options, depending on which body slot you want to use:
Lesser Wings of Flying, shoulders slot, 11,000 gp for permanent flight speed.
Winged Boots, shoes slot, 8,000 gp for 3 times per day fly for 5 minutes each use. Which should be more suited if you desire to fly in combat and not travel while flying.
Both are cheaper alternatives to a wand of fly (if you can), as it would cost at least 10,500 gp to craft, and would be spent after maybe 10 or so battles, not to mention the actions used to cast it on everybody. But they will hog the group's resources if you need to craft four of them.
To be honest, the spell you want is Mass Fly, but it should be available only at 13th.
Bring the flier down
Another, and much cheaper solution, is to bring the enemy flier down (unless for some reason you do require everybody to fly), and there are both mundane and magical ways to obtain this (but some are subject to gm fiat), such as nets, bolas, tanglefoot bags, Dragoncatch Guisames. And even spells, like Burdened Thoughts.
With an Harpoon, you could grapple and bring down a flier if any of you happen to land a critical hit while attacking with it. As per rules of grapple grapple, they are forced to immediatelly move towards your position.