There are really too many options to list specifics. This problem can be addressed through basic mechanics available in some form to most classes and available to all classes through feat trees (check out unusual weapons available through feats) and items. I’ll discuss each mechanic and give a level-5-or-below example of a power for each of them.
Anything that uses the ‘pull’ or ‘slide’ mechanic can help bring a flying creature down.
Most at-wills pulls have a limited range; the Swordmage’s Lightning Lure is great if the target comes within 3 squares of you (which, you’ll want to remind your GM, is 4 squares above the ground, as you count up from the top of the square you occupy).
Even if the pull or slide a power provides isn’t enough, there are items like the Orb of Forceful Magic that add squares to forced movement to make it more effective.
Encounter and daily pull powers often have greater range, or can add debuffs that may help your cause; consider the Wizard’s daily Visions of Avarice, which has great range. It pulls, immobilizes, and creates a zone that lets you continue to pull whenever you sustain it.
Knock him prone.
A prone flying creature falls. End of story.
Once you’ve got him down, keep him there.
Aside from grabbing –which is a lovely solution in itself unless the target has very high Athletics or Acrobatics; remember that more than one person can grab a single target, and he has to beat everyone’s defenses to escape– there are a number of powers which immobilize or restrain. The Runepriest Word of Binding is an at-will that immobilizes on hit.
Depending on your GM and the monster you’re fighting, you may be able to take advantage of the terrain: get under low cover that forces the monster to come to you, or engage in a place with low ceilings. Also consider insults: get him mad enough and he might “abandon methodical procedure for blind instinct,” as the Fifth Doctor said.
Do some searching for what you can use
Even if you don’t have a D&D Insider account, you can use their compendium to identify which powers and items have the words you’re interested in (pull, immobilize, etc) and it’ll tell you which source you can find it in. (If it says “Multiple Sources,” that probably means it started in the PHB1 and was republished several times in other books.) If you do have an account, it’ll show you the exact wording of the power/item with up-to-date errata, and tell you exactly what pages in which source material it’s appeared in.