Silver source ($4.50/foot) should last a loooong time.
http://cgi.ebay.com/99-999-5n-Pure-S...d=p3911.c0.m14
You might want to read the whole article, but here's an exerpt:
Making Your Own
The simplest way to control these factors is to make the colloidal silver for yourself. By doing this, you will not know exactly what is there unless you do laboratory testing, but you will have a pretty good idea. Without laboratory testing of commercial products, you don't know much more, because the quality control batch to batch is loose with most brands. Also, by making it yourself, you will end up with real "colloidal" silver, which is the product referred to in most of the literature.
If you are already making your own colloidal silver, please pay special attention to this section because much of the information you now have may be incorrect.
The simplest way to make real colloidal silver at home is by the "low voltage electrolysis" method. A few batteries may be connected to some silver electrodes and placed in a glass of water. This process will cause small particles of silver to be sintered off the electrodes and enter the water. This deceptively simple method is very easy to do WRONG, and most people who are making colloidal silver at home are making an inferior product.
It's In The Water
When you do this yourself, it is very important to control the purity of the water, because the purity of the water is one of the factors that controls how small the particles of silver will be. Only high quality DISTILLED water should be used. You cannot use purified or filtered water because it still has too many dissolved minerals in it. You cannot use deionized water because it doesn't conduct electricity well enough to start the reaction. Distilled water is just perfect to start the reaction slowly and let it proceed properly.
Another variable that influences particle size is the water temperature. The warmer the water, the faster the reaction will take place, and the smaller the particles will be.
The Best Is Yet To Come
The very best voltage for the reaction is 30 volts, because the electrodes run the cleanest at this voltage. If you have a small power supply, set it for 30 volts. If you are running on batteries, it is best to start at 36 volts (three 12 volt batteries or four 9 volt batteries) and let the batteries drain down from there. Holding the silver electrodes at a uniform distance away from each other yields a better product.
When 30 volts is applied across silver electrodes held uniformly apart in distilled water, a totally different event happens. First, the reaction proceeds very slowly. Often, for the first 15 minutes nothing seems to be happening. Then finally, a faint yellow mist will begin to form. Within a few minutes, the reaction will speed up, but the particles produced will be a golden-yellow as viewed with a flashlight. Using this method, 8 ounces of distilled water at room temperature can be made into a 3-5 ppm colloidal silver preparation in 20-25 minutes. Made this way, colloidal silver can cost under 10¢/oz to make. Electron microscope photographs of this product show a silver particle size in the range .001 to .004 microns. During manufacturing, the particle cloud is a golden-yellow. These particles will hang in the water at the level they are produced, and for the most part, will not fall to the bottom of the glass. This is what a "colloidal" preparation of silver looks like. After the particles disperse, the water will look clear again, but may turn a light yellow if the concentration is high enough and after the particles have become evenly dispersed.