I think you'd need a lot of plates and a fresh wild specimen. I'd start by taking stem base pieces of mycelium with a little substrate on them and let them grow out onto PDA, then as a back up technique blender some of the same material and streak out various dilutions. Look for colonies that are growing close to mycelium and don't show a zone of inhibition and transfer these to a plate of clean spore grown mycelium from your target mushroom.
I reckon it'd be a mammoth task - the number of simply commensal or just plain contaminating species will be high and you won't have the advantage of being able to use antibiotics. You'll also have to show that an isolate doesn't simply grow alongside the mushrooms but also improves their performance. A microscope and basic staining set might be a really useful tool.
Well worth a try though. A guy at work used to isolate symbionts from orchids - I'll see if I can get some tips at coffee tomorrow.
I got my acetobacter ( vinegar mother in fact ) by leaving a bit of wine vinegar exposed to the air and not worrying about flies getting it it - once it started to cloud I transferred it to cider and now have great thick mats of the stuff making me vinegar.
