You can get them from several sources like USA Midway, BC Precision, STM, etc. so you can shop around for the best price.
STM and USA Midway offer complete kits, tumbler, pins, everything to get started.
I initially bought 5lbs and only use half of that to tumble so since they apparently last forever you don't need a huge quantity.
As mentioned, if you choose to de-prime the shells before tumbling it's a good practice to check for pins lodged in the primer pockets after cleaning. I've never found any in small primer pockets but have found a couple in the larger primer pockets of my 45-70 brass. No biggie to remove.
A friend that shoots "Long" distance bench rifle competition stopped using stainless pins because he saw signs of microscopic peening on his shells and thought it might effect the hardness of the brass.
When you shoot at 1000+ yards as he does such little things are considerations but I'm just a plinker and have never seen any negative effect on my brass shells or my stainless 38sp stuff.
Ride Safe. Dr.Tramp..............