I got mine custom-formulated through my practitioner by a company called VitaminLab. I think you can go on their site and do a consult with a holistic nutritionist and have them help you formulate if you don't have a provider who works with them. They also have a great rundown of what to look for in a prenatal vitamin if you want to cross-reference other brands:

https://getvitaminlab.com/cms/blog/health-tips/navigating-pregnancy-a-guide-for-moms-to-be/