Anyone knows where can i find one for a dodge ram?  

fuel cells are usually universal in application, which means you can choose from hundreds and make your own mounts. I'm not aware of one built specifically for your Ram because there are so many spots to mount it. If you're looking for a cell to mount inside the bed of your truck than any one from JEGS or Summit Racing will work. Just pick the size you want.  

i just did some search about the fuel cells,i found couple that i like,but if they universal i would need to buy a sending unit/pump or find someone to custom make one for my stock sending unit right, if i get any fuel cell,my fuel gauge will still work with an aftermarket SU?  

fuel guages are set up on ohm readings. so you wont need a new sending unit/pump.  first find out what ohm rating your stock sending unit is. after you have found that info cross refence it with a aftermarket guage.  they are listed under ohm readings..all makes of vehicles take different resistance ohm numbers. after you have located your guage you want, it will have three terminalas on it .  a possitive,negative and a sender.  find the sending wire in your dash cluster or run a new one from the sender to the sender terminal.  run keyed power to possitive and ground to negative.  pretty simple!