I'm not confident in this idea but you can buy points with some airlines that allow transfer in to Marriott Bonvoy points. I would expect that the transfers in count towards the annual account limit and the transfer speed can be very slow but it might be an option. United have a 90% bonus at the moment, and that brings the price per mile down to roughly the same as buying Marriott Bonvoy points as they transfer in at a 1:1 rate.

Purchase United miles: https://buymiles.mileageplus.com/uni...d_landing_page
Transfer miles to points: https://www.marriott.com/loyalty/ear...s-to-points.mi

United appear to let me buy up to 248,900 miles (base + bonus) for ~$5k.

edit: ah! Limited to 100,000

"Convert up to 100,000 miles into Marriott Bonvoy points each calendar year at a 1-to-1 ratio. This benefit is only available for MileagePlus Premier members."

https://marriottbonvoy.unitedmileageplus.com/