If its just a slow leak then its probably easily repaired.
you can buy replacement bladders however you will struggle to find one for a wook 3 as its quite old now so no one will carry spares. However theres a company called airtime bladders who do orange universal bladders and then you buy whichever valves you need............however this will likely cost around 70 quid so it will be the same as what the kite is probably worth so not much point. The repair is the way to go!
heres a brief step by step of how to repair it.
1)undo the velcro tabs at the end of the wingtips.
2)if theres a zip in the LE to remove the bladder from, undzip that (if not then this means the bladder has to be removed from the wingtip.
3)now tie one of your kite lines to one of the bladder wingtips and gently pull the bladder out of the opposite wingtip.
4)if it has a zip then you tie kite lines to both wingtips of the bladder and pull through the zip.
5) untie the kite line/lines and leave them passing through the LE (theses will help you pull the bladder back in when you have fixed the bladder leak.)
6) now fix the puncture...........pump it up, put it in the bath, patch it with a bladder repair patch.
7) now to get the bladder back in. re-tie the kiteline to the tip/tips of the bladder

get someone to keep the bladder nice and flat/straight while you gently pull the kite line.
9) line up the valves
10) pump it up
Job done !