A very good place to find items for sale by owner would be Amazon. Or, E-bay is a very good site for this specific request. Other sites like Kijiji are reliable as well.
eBay would be a good place to start.
Craigslist.com is a great place to find condos for rent by owner for a good deal. Just click on "real estate - by owner" to avoid the real estate companies.
There is no "Best Place" but you could just play the game and while you are killing stuff for the quests you are sure to find some pretty good items, and the more higher level you are the better the weapons.
Craig's List is a good place to look for anything for sale by owner. Also look in the online classifieds for a local newspaper.
A good place to find items for couples who want to engage in teasing or foreplay is an adult online store.
Shopper.com is a good place to find a good deal on speakers and speaker systems. On this website, you can compare the prices of a wide variety of items, including speakers.
A great place for almost any used items including ultrasonic cleaners is ebay. There you can bid for items and often find very good deals on items you'd expect to pay more for.
The best place to find restaurant supplies at a good price is to go to the company Sam's Club. This company is a great place to find all kinds of needed items in bulk.
I find a lot of good deals on Amazon, sometimes you can get gently used items for even cheaper then a new one. BestBuy is also a very good place to get electronics.
Davidson glass items appear to be collector's items. There are a number of them listed for sale on eBay. A local flea market or rummage sale might be another good place to find them.
Craigslist is always a good place to look for used items. The downside is that you can't always find what you're looking for, since it just depends on what people are selling.
One can find an electricity cost calculator for household items by browsing local stores that sell household items. A good place to start would be a home depot or online sites such as Amazon.