http://www.veii.com/Parts-Store/USI/HARNESS-MEI-BILL-VALIDATOR-FOR-FAWN-3000-SERIES.html
http://www.dsvendinginc.com/p-7427-d4200708117-usi-mars-validator-harness-110-volt.aspx
http://www.dsvendinginc.com/p-8159-d4210981000951-usi-upper-validator-mounting-bracket-mars.aspx
http://www.dsvendinginc.com/p-8162-d4003038001951-usi-lower-validator-mounting-bracket-mars.aspx
Those should be all of the parts you need to mount a mars 2501/2511 validator in the USI machine. You will also need a standard power harness if you don't already have one for a validator. The data harness from VE is $28. The same harness from vendnet is like $87. You should also be able to get all of the parts from VE which is ideal for shipping but I don't know if the part numbers are the same and their website doesn't help. All in all, it's about $70+shipping.