I'm going to take a punt thinking your meaning a nylon toggle fastener opposed to a press stud fitting have a quizz on this wentworths link .. I've found the best way to fit tight clears or covers is to heat and soften them up in the sun to give it a little stretch first . Attach the ends first then secure the other fittings in between . 

https://www.whitworths.com.au/main_departments.asp?cat=152&Cat1Descr=sailmakers+accessories&Cat2=219&Cat2Descr=Fasteners

Shane O your clears would be tight from not being on the boat. The press buttons studs you will probably find that the pop rivets have corroded away hence the reason they are breaking off. Not sure if you can get stainless ones but they aren't to hard to replace.

In my opinion once you get the clears are on leave them on and take them off when you don't want them on. That way they stayed stretched. Only take them of for travel when you are doing long distances, as long as you can open the front up for the short distances like from your house to the boat ramp. That is what I use to do on my old boat and the bloke who brought it of me does the same (my neighbour across the road) and they have lasted over five years. 

where can u get replacement ones 

 Whitworths have the studs. Couple of different sizes so measure yours and get both male and female. Make sure you also get the small tool to insert inside the stud to assist with hammering and flaring the inside of the stud, makes life very easy and is an easy repair. From memory the studs are about 50 cents each.

Yeah what Chris said, leave in the sun on the back yard for a few hours to heat up works a treat. I also found Clark Rubber stores stock those push studs as well.