As far as my knowledge goes in this matter, here are the following options for you:
Equity Trust Company offers both self-directed Roth and traditional IRAs. Low account fees start at $135 per year. Supports a wide variety of alternative investments like real estate, private companies, precious metals, etc. Over $25 billion in assets are under custody.

IRA Services Trust Company is another low-cost leader in self-directed IRAs. Annual fees start at $75 for basic custody services. Access to diverse alternative assets. Over $16 billion in retirement account assets were serviced.

Broad Financial offers competitive pricing with account fees ranging from $97 to $295 per year, depending on the services needed. Specializes in real estate IRAs but supports other assets too. Has over $8 billion in custodial account values.

Kingdom Trust Company: Account fees range from $100 to $250 based on the choice of service. Offers flexibility to invest in traditional as well as non-traditional assets. Does not charge transaction fees, which adds to the overall value.

New Direction IRA: Account setup fees start at $100 and annual fees start at $295. A wide menu of alternative investments supported, including private companies, startups, loans, tax liens, etc.