Funding my IRA

Can I rollover a 403 (b) into a Self-Directed IRA or Solo 401(k)?

Yes. 403bs are very similar to 401(k)s in the way they are treated by taxes.

What is different about a 403(b) compared to a 401(k)?

Usually, government workers, teachers, or charity employees contribution pre-tax, and sometimes with post-tax Roth Dollars. 403(b) plans are just a 401(k) for public employees that do not have a pension. 

So 403(b) dollars are exactly the same?

If you contributed pre-tax, (defer payroll taxes and pay taxes in retirement) yes, they are. This could be rolled over to any Traditional IRA, Workplace 401(k) SEP IRA. 

You can check the IRS rollover chart for more information. https://www.irs.gov/pub/irs-tege/rollover_chart.pdf