Seal ink can be purchased in several formats. For everyday use ink can be found in small metal lidded containers. The advantage to this format is that it is inexpensive and convenient to carry with you. But I know people who bought these at tourist shops only to find the ink dried out and unusable; you can’t know how long it may have been sitting on a shelf before purchase. Your chances are better if you buy your ink from an art supply store as there may be more turnover and more concern for quality.
Another format is sold in lidded ceramic containers. Outside China seal ink is usually found in white porcelain containers with a blue dragon design on the lid. In China there is more choice available in shape, size, and color of container. This ink may be of high or low quality—usually a factor of price. Avoid the tiny ceramic containers (about an inch or so in diameter) which are too small for normal use.
The third format, and the one preferred by artists, is seal ink sold in small packets. These are usually only found in art supply shops, although they are also available online. After choosing the color of ink you then buy as many of these packets as you need for the size of your container of choice. This ink will usually be of better quality than that found in the metal tins. There are many brands available but the Xilingyinshe brand is best known.