It just takes time. Could sell gems from Mining they go for like 40k each, pebble companions, selling unwanted items or even just using MS to get Pebbles. Could take a few days still though
Actually, gems sell for 4k pebbles each. You can get them at every level of mining. Since your achievement level is low, I'm guessing you haven't been playing for long and probably don't have a lot of things stockpiled. So selling trinkets acquired from campaigns and food you cook in inventory, completing pebble rewarding achievements and daily activities may be more your options.
