Calling people up on the 'blower is not the way to best go about it, methinks; keep your eyes open all the time and when you see the guys working wait for the right moment to approach them (tea break, smoko, or just a quiet moment). Don't bother them when they're busy, machinery running, climbing prep. etc., and when you do speak to them be as open and friendly as you can; explain what you do and what you waqnt, don't go on at length unless you're invited to, and it's a good idea to have a spoon or some artifact that you've carved or a work in progress to show them - I used to carry a couple of small bits with ne before I got to know them all, it's amazing how these little "ice-breakers" can stir interest in what you're doing...............and don't be shy of offering to carve 'em something small from the wood you get from them.
The golden rule though is to never try to make them think you know more than you do - good tree surgeons know a lot of stuff, and people who know their stuff will generally take pleasure in giving you advice on timbers and will also be more likely to put you on to other sources if you've presented yourself as friendly, intelligent and interested in what they do, and willing to be educated. They also know the smell of BS as well as you know the smell of coffee.......................Above all, don't wast anyone's time!
I hope this will be of help to you.