There's a giant dragon named Greyroll in the region of Caelid at the east of Limgrave. You can access it through a secret portal via the third curch of Marika in Limgrave. The region is high level and the fight is supposed to be very tough since while Greyroll can't move, she has multiple smaller dragons around her and a mountain of HP. However, you get behind her where she and her dragons can't reach you, take a bleed weapon and hit her until she dies (bleed do a percentage of HP so it can go very fast). With a bleed weapon, it takes around 3 minutes or so (you have a fort in the south of Limgrave where a Godrick knight drops an Ash of War with bleed effect you can put on a weapon or you can take a flail with bleed effect somewhere else). She will drop 50,000 runes in one row. However, if you happen to have consumed a Gold-Pickled Fowl to (shortly before killing her), she will drop 80,000 runes. You can find one relatively easily right next to the first site of Grace in Limgrave.
There are other interesting details so I can't go through exactly all the steps but you can easily find videos on YT if you want. Also, the portal from Marika's church leads you to an area with a very dangerous boss (avoid him for now) and several high level enemies who are very strong BUT drop more than 1000 runes each so if you're cautious and not too greedy, you can use that as a farm ground