Our doctor resisting giving referral for second opinion

Hi, my mom was just diagnosed with stage IIIB non-small cell lung cancer and I'm looking into getting a second opinion from another expert or two, mostly to confirm her oncologist's treatment plan and to explore other options like clinical trials or anything else that might give her a better chance to improve or even survive. 

I've been researching lung cancer specialists and have identified a handful of renowned experts I'd like to visit to see if they know of anything promising beyond standard treatment. The problem is our primary care physician isn't aware of any of them and they're also out-of-state. She said she would do a referral, but needs to know specifically why and that I need to give an acceptable reason. And that if I was interested in clinical trials, I should ask around at various departments instead.

Honestly, I don't know how this process works or if I'm going about it the right way. But I do want to at least see what these experts have to say regardless whether insurance covers the consultation or if we pay for it out of pocket. It's certainly more reassuring and less of a hassle than having to investigate and evaluate all these other potential treatment options myself. 

Anyone have some suggestions or experience with this?  Thanks in advance!