Does anyone know if my out of state Texas teacher retirement/pension is taxed in the state of Missouri?

If you now live in MO then your income is subject to tax by the state of MO, no matter where the income came from.   It is not relevant that your pension comes from your former TX job.   Enter your retirement income when you prepare your federal return.  The information will flow to your state return.   The TurboTax software will use your state's tax laws on the MO return.

States that may tax retirement income from IRA's, pensions, etc.:

AL, AR, AZ, CA, CO, CT, DE, DC, GA, HI, IA, ID, IN, KS, KY, LA, MA, MD, ME, MI, MN, MO, MT, NC, ND, NE, NJ, NM, NY, OH, OK, OR, RI, SC, UT, VA, VT, WI, WV, 

This is a “general” list.  Some of these states have complicated laws that exclude certain types of retirement income, or provide certain income exclusions and age deductions.  This list does NOT refer to Social Security benefits.