It is given out by Lord Searfire in Solusek Temple. For him to talk to you, you must have amiable or high faction with him.
To raise faction there are three things you can do. In South Ro, you will find an Orc and an Erudite, standing in a tent by the southeast wall. To raise faction with the Temple of Solusek Ro, either give the Erudite 30 gold or give the Orc 4 dervish Cutthroat Rings at a time. These rings are obtained by hunting in North Ro at the Dervish Camps. The Dervishes, which are at these camps and drop the rings are level 10-16. The other way to raise faction is to hunt shadowmen in West Commons or South Karanas. But if you are not a high level, they are hard to solo and single pull. Once you have high enough faction to talk with him, he will give you a Sol Cauldron. This is what you will be combining your ingredients for the bar of Ronium. In this Cauldron you must combine, Soil of the Underfoot, 2 Enchanted Bars of Platinum, Sands of Ro, one Mistmoore Granite, and one Melatite.
The Soil of the Underfoot is obtained by giving Priestess Ghalea in the Kaladim Cleric Guild, 4 fairie dusts found most commonly in Greater Faydark. The problem with this is that killing fairies hurts faction that you need to have to complete the Epic Quest later on. To get around this, you can either buy the dusts from other players, or have an alternate kill them and transfer the dusts to your Ranger. After you have the dusts, there is another faction problem. Before handing the dusts into the Priestess, you must have amiable faction and if you are a Ranger, you faction will probably suck with the Kaladim Cleric Guild unless you have raised it somehow before. To raise it, you must give the dwarf at the Paladin Guild, bone chips until your faction is sufficient. Hand in the dusts for the Soil of the Underfoot.
The Enchanted Bars of Platinum are found by giving a 34 or high level enchanter two bars of platinum which can be bought at any jeweler store for around 100-110 platinum depending on your charisma.
The Sands of Ro drop off of Sand Giants in Oasis of Marr. Sand Giants are approximately level 29-35. You can either get in a Sand Giant group or try and solo them if you are high enough level. You can also buy these from players and sometimes even from vendors such as the gypsies if some Sands of Ro have been sold to them recently.
The Mistmoore Granite is found off of Gargoyles (Sneering, Jeering, Flouting, Leering, and Grinning) in Mistmoore, but mostly just the Jeering and Leering types. They are found in the castle area, especially right at the entrance of the Ballroom and outside the Castle close to the ramp toward the cemetary area. You will need two Mistmoore Granites, one for the bar of Ronium, and one for the actual quest.
Melatite can be bought off of jewelers or found off of Clockworks within Solusek A.
Combine your Soil of the Underfoot, the two enchanted bars of platinum, Sands of Ro, one Mistmoore Granite, and your Melatite to make your Bar of Ronium.