First, does your crank have bolted on chainrings? Second, do you have any particular reason for wanting to stick with the same crankarms?

I ask those questions because chainrings are generally not very cheap. To replace all three chainrings, or even just the two largest rings, on a medium to low end crank will probably cost you as much as buying a whole new crank. If the bike is low end, you can get VERY cheap 48/38/28 cranksets which are great for commuting (in my opinion of course).

This is nothing special but it'll get the job done:
http://www.universalcycles.com/shopp...&category=2637

This is about as cheap as it gets:
http://www.niagaracycle.com/product_...oducts_id=6611

Here's something nicer with slightly smaller rings:
http://www.speedgoat.com/product.asp...=32&brand=1004

Do note that you may need a new bottom bracket depending on which crankset you go with. The above are all square taper so if you have a different interface and don't want to swap, you'll need to find something else. Your current front derailler should have no trouble with a 48 tooth big ring. If you wanted to go with a 50 or 52, then you might want to consider a road front derailler though that brings it's own set a of issues, and for that reason I'd recommend sticking with a 48. A 48/11 combo is bigger than a 52/12 (common road bike top gear) anyway.