You can begin immediately to contact any or all of these members at your convenience. They will also be updated with your new member listing, so they may contact you before you contact them. There are many ways to get picks and even a few ways to get them for free or very little cost. The best way to get free picks is to catch them at concerts when the bands and artists throw them out to the crowd. If you aren't fortunate enough to be seated close enough to the stage for this to be a posibility, you can write to many of the artists through their record labels, management or fan clubs and many of them will send you a pick if you enclose a self-addressed stamped envelope.
Many bands also sell their picks on official websites or through their fan clubs.
There are also the music memoribilia dealers who almost always sell picks.