As a Pakistani citizen you need a visa to enter Cambodia and Laos. Also you may not be eligible to apply online for these both countries.
You are not eligible to apply online for a Cambodian visa. Therefore you need a proper visa from a Cambodian embassy or consulate.
Source: Kingdom Of Cambodia
Further since you have a valid visa from Thailand, you can apply for a Cambodian visa from The Royal Embassy of Cambodia in Thailand. All details regarding visa and application procedure can be obtained from Cambodia Mission Worldwide, and simply contact the royal embassy of Cambodia in Thailand.
Moreover your travel agent saying you don't need any Cambodian visa is totally wrong. You can also visit Visa Policy of Cambodia to confirm that you definitely need a visa and are not eligible for an online visa either.
You are also not eligible to apply for an online visa for Laos.
Source: Laos Visa On Arrival
Similarly you have a valid visa from Thailand, you can also apply for a Laos visa from Thailand. You can contact Laos embassy in Thailand from Laos Embassies And Consulate Abroad and can get all requirements regarding the application procedure.