You will have to see which firms let you invest in low-cost index funds with your employer's 403b, and what those costs are.
This.
We are in the process of moving my wife's 403b away from The Legend Group with it's facepunch worthy funds that have ERs between 0.9% and....wait for it....wait for it....2.79%. Yes, that's right. It's almost past a facepunch and closer to groin kick.
Anyway, we are dispatching that shit post haste and moving to Aspire. They have an open platform and allowed her to add the Holy Trinity of Vanguard Admiral Share funds - VTSAX, VBTLX, and VTIAX (as well as VTABX, although we may not use that one). Not quite as good as zdrave's experience, as Aspire charges a 0.15% (15 basis points) asset management fee, as well as $40/year management fee. Compared to the prior ERs, though, even paying the asset fee will literally save us thousands of dollars per year in straight expense, and more if you count the potential gains by keeping that extra money invested. I think other Mustachians have mentioned Valic as having a similar setup, but haven't researched them as they are not an approved provider for her district.
403b plans seem to be one of the last holdouts of nearly unregulated greed, so to an extent it's finding the least of the evils. I'd look for anything that has low-cost index funds with no asset management fee (basis points/percentage) and, failing that, go for an "open brokerage" plan that will let you add funds a'la carte even if there's a small asset fee assigned.
Best of luck...