There are several different places you can get your screen replaced. However, if your ipod is still under warranty you can get it exchanged/replaced for free. Here is a site that offers inexpensive ipod repairs...
Apple can also repair your ipod for around $100 if you dont have a warranty; just call and make an appointment for a repair and then take it to a nearby Apple store.
BUT you may not have to get the screen replaced if you can dry it out. Have you tried sticking in a bowl of uncooked rice for a few days? Or Silica Gel Packets...
If the water is truly stuck under the screen, don't use your ipod as this can damage it further, and take it in to get the screen fixed. Hope that helped .