Tracey,
Usually you can hire any PI for anything that you want him/her to investigate. You set the parameters and communicate what exactly you want that individual/firm to do.
I know in the US, they're listed in the telephone directory. The ads tend to state what their speciality or expertise is in. Sometimes police departments can give word of mouth referrals, esp. since many PIs are former cops. Hope this helps a bit to get you started.
good luck,
toodles