The attenuators probably have 2 functions:-
1.Prevent the Tx output of one AP from damaging the Rx of the next AP.
2.You can use the attenuator to simulate distance.
A 70dB attenuator would give you the equivalent distance of approx 0.02 miles at 2.4 GHz.
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You should be able to find a variety of microwave attenuators at new/used Test Equipment stores.