How to get SQL Server available alerts infomation ?

Alerts information is stored in the msdb database. Here is a sample query to get a little information about alerts on your system:

SELECT  SA.name AlertName,
        SC.name AlertCategory,
        SJ.name JobName,
        SO.name OperatorName,
        SO.email_address OperatorEmailAddress
FROM    dbo.sysalerts AS SA
LEFT JOIN dbo.sysjobs AS SJ ON SA.job_id = SJ.job_id
LEFT JOIN dbo.sysnotifications AS SN ON SN.alert_id = SA.id
LEFT JOIN dbo.sysoperators AS SO ON SO.id = SN.operator_id
LEFT JOIN dbo.syscategories AS SC ON SC.category_id = SA.category_id

Be aware that the designer of the tables in msdb (not just alerts) decided to store date and time information as ints. You will need to consider that when wanting to do date comparisons or conversions.