I'm sorry that you are going through this, no one should ever have to feel betrayed.
If you are feeling the need to spy on him that strongly then your intuition is probably correct. Do you REALLY want to see the details? Sometimes that can be very damaging to your psyche.
It's actually illegal to spy on people so you could cause yourself double grief:
In summary, there are very strict laws which apply to electronic communications such as emails, computer communications (e.g., chat rooms, instant messages) and telephone calls. Spouses who attempt to spy on a cheating spouse using surveillance equipment should be aware that they could find themselves on the wrong side of the law.
Have you tried talking to him about it? If he cant convince you that he's not cheating have you tried a counselor? Honestly, you should go that route first before playing double O7.
Sorry, I dont know of any equipment but I'm sure you can shop around online and find something... or maybe just head over to Radio Shack. Be prepared to sign a disclaimer when you buy the equipment legally releasing Radio Shack from all liability.