Where to buy an immersion heater?

The Electric Brewery is a very popular source for building electric homebrew setups when using steel/aluminum kettles. They sell prebuilt heaters and they also give a list of all the parts they use so that you can buy them separately elsewhere (e.g. Amazon) and put it together yourself:

http://www.theelectricbrewery.com/heating-element-kits

I personally would go this route when using electric heating elements because it's safer. With heating elements inside plastic coolers, you really need to know what you're doing or you could melt it and/or leach chemicals from the plastic if it's not rated for water hot enough. But if you make sure you can keep it in the safe temp range (sparge temps), 1000W or 2000W would be enough to get you to sparge temps.

Camco - the brand the Electric Brewery guy recommends - also makes 1000 watt 120V and 2000 watt 120V heaters.

Or search the web for 'homebrew heat stick' and you'll see lots of other ones. Heat sticks are often used to supplement heat from a stove for full boils and people often want them to plug into 120V outlets in the kitchen, so they don't need to be that powerful.

or a bucket heater like this Allied Precision Bucket Heater (says Marshalltown, made by Allied). Note the bucket heater is not necessarily safe for potable water, though hundreds of homebrewers use it. I used to use it to supplement my boils, but found that it had too many spots that could not be cleaned, so until I can take the rivets out and take the steel shield off, I won't use it anymore for that. But for heating water, it should be ok.

In summary, you can get everything on Amazon. I'm sure you could also get it at a hardware store or plumbing supply store as well.