@moondafox 13 moonstones is less than a week worth of gameplay since you can get up to 25 shards a day from just playing, plus more from doing the weekly activities. I would suggest trying to grind it out. You should also check out your Achievements if you haven’t recently. Some achievements reward MS, but you have to manually claim them on the Achievements page. If you have pebbles or items you can liquidate, you can sell them to Marvin and buy any extras you need on the flea market. I make sure to check in somewhat regularly to make sure there is MS supply, capped at 120,000 pebbles per MS so we don’t experience as much price chaos, and others frequently come in with even lower prices if you check back regularly.
The user events and giveaways section mentions that it is for hosting events and giveaways, so I’m not sure you’ll have much luck with this kind of request here, but I wish you the best on your MS grinding journey.