Sorry to hear your husband is betraying you but to find a private investigator you'll have to find one locally through an online search for private investigators in your area.
If we knew the state or location you're in we good let you know of some good private investigators in your area that you could hire.
Private investigators cost from $40.00 to $200.00 per hour and make sure that what you hire them for that they're not doing anything illegal in doing the private investigation on your husband.
Good luck though.