If it's your phone call your carrier and ask if they keep texts and if they do how you can get them.
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Subpoena your records from the phone company, and from the person who was texting you, and his/her phone company. If you have a lawyer, he/she should be familiar with the subpoena process. Also, if these texts are evidence of a crime, you can report it to the DA and the DA will subpoena the records.