Returning player having issues

Hi guys just to be clear I'm not begging for your hard earned caps or items. After being away from fallout for about 18+ my build an weapons just don't seem to be enough lol. I'm trying to get hold of an elders mark. I don't have any of the legendary mods so I need a stock version. Roughly how much so they sell for. Or when will the beast of burden mission come around again. I've been checking everyone's vendors but not having much luck. Thanks