You could also write a custom method like

again, the above only really works when selecting 5 from 15 - it's picking balls from a bag, seeing if it's been chosen and then PUTTING IT BACK. if you were picking 5 from 5 you'd end up picking a lot of repeats before you got that final ball (and consider how terrible picking 1000 from 1000 would be)

the knuth shuffle is like picking balls from a bag and not replacing them, it's linear, scales well. the inefficiency here is that you shuffle all 15 balls when you only want the first 5.

so combining the two (it's a partial knuth shuffle)

select a ball at random from the 15 available
replace the 15th ball with the one just chosen
select a ball from the first 14 (effectively ignoring the one just chosen)
replace the 14th ball with the one just chosen
select a ball from the first 13
...

your 5 chosen balls are then the last 5 in the set.