OEM Parts - I usually google the part number and go with whatever is the cheapest, MUST BE OEM.
Aftermarket Parts - I look for all vendors who offer military discount, if prices are all the same I tend to go with the actual manufacturer.
Maintenance / Servicing - Hard to beat the prices on oil at Walmart. As for filters, Genos has GREAT customer service, their prices are good, plus they have military discount which they apply to your account so its always there.
Non RAM Maintenance Parts - I have had great luck with Rock Auto and as mentioned above... Google. Take a few minutes to research what you are looking for and you can always save a few bucks.
With that said I have had great experiences with these companies who focus on these trucks: Genos Garage, CJC Off Road, Thuren Suspension, I have reached out to these companies in the past with positive results.... HIGHLY RECOMMEND.