I'd want to know why the starters are failing. Can you give more info?
1 What type of ballast(s) is fitted, is it wirewound or is it electronic? Make and model number?
2 How many ballasts are there and how many tubes are there? What type are the tubes or what's their model number?
3 When the starters fail what happens? Have you dismantled a failed starter to check out the damage, if so what did you see?
4 How long do the starters last before they're kaput?
5 What is the make and model number of the starter?
Any chance of a photo of the inside wiring of the fitting?
It's a bit of a weird one because the starter isn't wired as part of the fitting it's only wired to the tube itself so I can't immediately think of a reason it would prematurely fail apart from those already mentioned.