You can obtain a Summary Plan Description from your payroll department or Tax Deferred Solutions which will list the investment providers you are allowed to use. You cannot use an investment provider that is not on the 457 platform. The investment providers are not the same as the 403(b) providers.
If the provider you wish to use is not on the platform you cannot add the provider to the plan. Only your employer can add a 457 provider to the plan. Typically, an employer will require that there must be a minimum number of employees interested in using the same provider in order for your employer to add a new provider to the platform.