They sell Ro/Di water at Walmart here on Long Island. ..think it's around .88cents a gallon..... Surprised they don't have it at your Walmart. A 2 gallon water change is not much unless you have a 10 gal tank...
Sorry , did you mention the size of your tank???
+ 1 i have a 30 gallon and do weekly 5 gallons, and i still don't think that's enough. aren't you in sayreville or east brunswick? hit up aquatic obsessions, captivating coral, and red sea. they are all pretty much in our area. rodi ranges from $.50 - $1.00, and saltwater is anywhere from $1-$1.50, depending on what salt these vendors use.
To answer your question, yes 2 gallons a week in your 40 gallon tank is a little low but not bad. It is more than I change and I also have SPS and LPS.
Stop using spring water. It has trace elements and minerals that may not be healthy for a reef. Safe drinking water is anything below 400ppm. Most tap water has less than 1/4 of that. Spring water may fall in the middle somewhere, which means "no bueno". If you're going to use bottled water, use distilled water. Or, crunch some numbers comparing yearly bottled water costs vs. one lump sum for an RO/di unit. Don't hang it up anywhere if it bothers them. Keep it stored until you need to make water. Fill up your bucket and store it away again. Just something to think about.