Hi,
I have never found a service that provides alerts for Parabolic SAR so if you do find one let me know as I am sure that their are many people who would like to use this service.
I do know that at
Easy financial/stock chart - [finance/stock chart with full source] you can 'scan' any stocks and any of the exchanges using various indicators (including Parabolic SAR) for valid entry signals e.g. when Parabolic SAR has just reversed but you will find that unless you are using easychart or trade with a broker that utilizes easychart as part of their platform the signals will differ between platforms and timezones (like Metatrader for example).
As far as I know you CAN get Metatrader to send you an alert from Parabolic SAR (although this is not a 'standard' feature but I seem to remember seeing somewhere that someone had 'written' a 'custom' Parabolic SAR indicator to achieve this).
Hope this helps.