A DIY store is a "Do It Yourself" Store. He's talking about a Home Depot, Lowes or a plain Hardware store.
You can get them online from McMaster-Carr who I believe I am supporting almost by myself when I look at the engineering groups purchases from the last quarter.
http://www.mcmaster.com/#91169a170/=agkzaxThis is a good choice. It's un-plated, it's steel (not stainless), it has a black oxide finish and it is a good thickness and diameter for a Uberti Arbor hole (arbors run around Ø.42. I don't know if Coffinmaker lives where metric is the standard, but after comparing a few sizes, a metric washer like he specified is actually better for this application. The thickness of the English sizes in the diameters needed tend to run too thick, you can drop a screw size from #10 to #8 but then you get less contact area.
The price is $2.26 for 100. There is an alternative which is a wave washer Part # 90134A029. I've tried those but they are $11.25 for 50 and they are only .01" thick. But if you want a spring fit it works well (you lose them a lot if you load off pistol).
Coffinmaker is using the split washer as a spring to take up the difference in thickness needed to bottom the arbor in the barrel channel. The washer is 1.1mm thick which is .043" the gap on a Uberti between the end of the arbor and the hole probably runs around .05".
Good call Coffinmaker, I hadn't considered metric.
You can buy online and have your parts tomorrow, the shipping may get you though they have charged us $5 for little things and at others $1.75. You can call your order in if you wish and thy will probably tell you the shipping charge.
Regards,
Mako