I also have mine with fidelity. They have many index funds that are comparable to vanguard admiral class shares. Here's a good place to start:
https://www.bogleheads.org/wiki/FidelityThey also allow incoming IRA rollovers which could be helpful if we ever aren't eligible for roth IRA contributions and want to clear out traditional IRAs in preparation for doing a backdoor roth contribution.
Unlike vanguard, fidelity doesn't offer a roth solo 401k which might be a disadvantage depending on your feelings about roth vs traditional (I'm in the traditional beats roth camp). I think etrade might be an option that both allows incoming rollovers and roth, but I think there may be more trading fees (not sure about that though).
Vanguard would not allow me to rollover my firm-401k to a solo-401k. Fidelity did allow me to do that and so that's what I did. I have it invested in an index fund (it's only $13k so its all in one fund).
Too late now, but for others who might be thinking the same thing, this may have been a mistake. I believe employer sponsored plans (and IRA's rolled over from employer sponsored plans) have better liability protection than solo 401k's. This might depend on the state, and I don't know how big a difference it would really make, but it would be worth looking into for anyone thinking about rolling an employer sponsored plan into a solo 401k.