Anyone had their tissue (from surgery) examined for 2nd opinion?

I believe the way to proceed would be to do your own research to find a well regarded academic institution where second opinions are done. Ask them. I believe it would be an institution to institution transfer rather than you picking up your sample. Have you shared your interest with your surgeon? I think that would be a reasonable request when cancer or another serious condition has been diagnosed.

I did consider getting a second opinion on my initial biopsy. I'd done that when my mom was diagnosed with breast cancer and wanted a second opinion besides the one from her local hospital. Because she was in an HMO, I researched her coverage and learned her local HMO was linked by referral to a more comprehensive hospital in another city. That hospital had a more specialized breast cancer program and so requested that slides of my mom's initial biopsy be sent to that hospital. Her primary care doctor and obgyne doc did not know this option even existed. No surprise since docs don't have the time to know the details of insurance.

No one suggested or explained the process of a second opinion for biopsies. I learned it as I went along. As I recall (this was several years ago now), I had to provide the information to the local hospital, sign some papers, and call and make arrangements with the hospital where the slides would be sent, etc. This was all done pre-internet days so it's a smoother process today. I think it was probably all handled by fed express or UPS for biological specimen handling. I don't remember if I paid or it was covered by mom's insurance plan.

We got the second pathology results fairly quickly. They confirmed the initial biopsy findings. I found this reassuring.

Flash forward to my own situation. After my initial biopsy and pathology report, I seriously considered getting a second opinion. I researched online and found I could have my slides sent to Johns Hopkins. I'm pretty sure all major breast cancer center hospitals, such as Mayo and MD Anderson, also will do second opinion pathology. You'll probably have to do some paperwork.
https://pathology.jhu.edu/patient-care/second-opinions/send
Based on my experience, I'd deal directly with the pathology department of your hospital. If you call them, they know exactly what you will need to do. When I asked my own doctors involved about how to do this, they seemed unaware of the procedure. That is odd because I'm sure I'm not the only patient who's inquired about this.

In the end, after all my own research, I decided not to get the second opinion. I had pretty high confidence in the pathology report because of my hospital's reputation. But I do think second opinions are wise since the diagnosis and treatment to follow depend heavily on that biopsy report. I think I heard that more than one pathologist interprets the biopsy at my hospital. I'm not sure if I heard that or imagined it, but I think 2 independent pathology readings is so important, it should be required.