If you need replacement toner cartridges your best bet is to start online. First, go to eBay or amazon.com and enter the long version of what you want. Look for your model printer, the cartridge number, the year… etc. The more specific your search the more specific your results will be.
Unfortunately this step is not always enough to find you the best deal out there. Especially if you want a quality cartridge or if you want to buy in bulk. You may need to search specifically for cartridge numbers, as some fit to multiple model printers and including those in your search may be excluding good results. You may need to search for items that ship specifically from the United States as well as many generic, low quality ink cartridges are posted on eBay-type sites from factories and outlets oversees. Some of these outlets may actually be a good investment, but it is probably best to order a trial product before you place a large order.
If you want genuine printer equipment at a discount price, try searching for companies on Google. Larger companies with stocks of name brand toner will usually have their own website and discounts for customers ordering large stocks of toner. If you find a good deal do not be afraid to contact their customer service and ask about discounts for bulk orders or other specials they may be having. It can save you money and time later on.
Dont be afraid to take some time and really dig into your search. Your persistence will be rewarded.