If you are in the Uk there are plenty of places that do them. A quick google reveals places such as this http://www.custom-fasteners.co.uk/ac...ad_Screws.html
Screw fix used to sell packs of 50 for under a tenner but sadly the currently do not stock them. They were 100mm long but we used to just cut them down.
I am sure somewhere online near you will sell them. No need to buy from a ws dealer if they don't have them in stock or want the hassle of ordering just 2 in for you. Just make sure the ones you buy are good quality marine grade stainless or they will rust to smug after the first outing.