not many spare parts shop sell dot 5. dot 4 and dot 3 are more common. Normally the brake fluid will reach the wet boiling point after >6 months, so its best to get it replace after 12 months.
for easy replacement, its best to use 5.1 if you are planing to go dot 5 direction. cause dot 5.0 its silicon based and will not absorb water, so changing from dot 3 / 4 will need to have many round of flushing. 5.1 will poly base, like dot 4 and 3
some info about brake fluid :-
http://www.swedishbricks.net/700900FAQ/Brake Fluid Comparison.htm
for easy replacement, its best to use 5.1 if you are planing to go dot 5 direction. cause dot 5.0 its silicon based and will not absorb water, so changing from dot 3 / 4 will need to have many round of flushing. 5.1 will poly base, like dot 4 and 3
some info about brake fluid :-
http://www.swedishbricks.net/700900FAQ/Brake Fluid Comparison.htm