If you want to buy it online, there is a source here.
http://www.letterexpert.co.uk/WrittenWarningTo AnEmployee.html
There is no particular reason to have different template for different reasons. A 1st (2nd) (Final) written warning are three templates that could be all very similar apart from the heading. All of them are likely to include a final sentence along the lines "Failure to improve your performance in the area described above is likely to need to further disciplinary action which may eventually lead to your dismissal".
The specific issue (failure to call in when sick) must be a company rule - often it is put in the Employees Handbook.