Comcast uses and only supports the following splitters
Extreme Broadband Engineering
None these can be purchased locally, but are available from many online sources. You should be able to pick up 2-way splitters at your local Comcast office, or flag down a Comcast tech and he might be able to help you out if you need 3-way or 4-way splitters. Examples are HERE
Monster are not recommended and are over-priced. A Comcast tech would remove it if he found it.
Look for splitters that pass 5-1000mhz. Avoid splitters passing up to 2200mhz, which are designed for satellite installations.