Look up Stan Pro window channel, you can order it from somewhere. The
part that will disappoint you, however, is only a year after you
replace it, it will look like hell again. That felt just doesn't
last. If I were to do it again, I would seriously consider re-working
the windows all together, either solid glass panels that don't open,
or some other design. Be sure to check your frames for holes when you
have the glass and channel out, that was where my leaks were coming
from. The old screw holes that the channel was secured to, they had
gotten bigger with time and water was able to seep through.
I think Stan Pro or someone else makes plastic channel that's cheaper
and might actually weather better, assuming the plastic is just soft
enough that you can do without the felt altogether.
Good Luck
Tom in Florida