Most bikes that came with those only came with them on one side. It's not necessary to have them on both sides, although I've been known to install them on both sides. I'm not sure where you can buy them, maybe someone like www.loosescrews.com (you might need to email or call them so they know what you're talking about). I wouldn't slide the axle all the way to the rear of the dropout, your shifting will likely suffer.

P.S. Does the bike have an integrated derailleur hanger, or an adapter claw? If it uses an adapter claw, I've found that with the adapter claw in place and the spacer positioned as far back as it will go on the other side, if you pull the axle all the way back against the adapter claw and the spacer, all is usually good in terms of wheel alignment-