Consider the following alternatives when looking for the best prices:

Newspapers
Most newspapers have a classified section for building supplies or materials. This can be a great opportunity to pick up left over materials from a job. If you see an ad for any material that appears to be bathroom related, call the seller to see if they have any of the items that you need. Even if they do not have what you are looking for, they may be able to offer some additional resources from their circle of professionals.

Seconds and Surplus Stores - Large Supply House
If you live in a larger metroplex, this may be an option. In Dallas, there is the Habitat for Humanity Home Improvement Outlet Store where people donate extra building material and the store sells the items and redirects the profits to building homes for deserving families. Another resource in the Dallas area is Seconds & Surplus where you can purchase discounted building materials.

Large supply houses may offer discounts on discontinued items, floor models or open box returns. This can be a great resource especially for bigger ticket items. And always ask for their best price - no matter how much the item has already been discounted.

Just be sure to compare prices against a reputable source, as discounted doesn't always imply the best price available. Also, take your drive time and fuel into consideration. Spending an entire day driving around the city can be more expensive than shopping online and having a bathroom vanity delivered to your front door.

Internet
Once you find the item(s) that you are most interested in, compare prices across multiple sites for the same or similar product. Be sure to take shipping costs into consideration when comparing prices from website to website. If you find a site that you would like to purchase from, ask their customer service if they will price match on like items. If you are working with any professional on your remodel project (general contractor, designer, etc) ask the web based company if they offer trade discounts.

Be sure you are buying from a reputable company. Buying large ticket items online has become very simple, but freight damage can occur from time to time. You want to be sure the company you are buying from will be there if you have any problems. This is the best alternative though for those that have limited resources within reasonable driving distance.