Sts is better (silver thiosulfate)
You can buy colloidal silver just about anywhere. There's lots of sellers on amazon. Don't get dupped into buying one 'for weed' it's all the same stuff (maybe different ppm) just anything 'made for cannabis' costs 10x more at min. Lol.
As h&s said you can make your own colloidal silver and sts. It's easy. If you can mix nutes you can make it.
The pro with STS is once you buy the silver nitrate and sodium thiosulfate anhydrous (or pentahydrate) you'll have enough for many many many breeding projects. Just mix what you need and store the 'a&b' seperate and mix em up when you need it. Mixed soultion will stay good for a month or so (kept in a dark cool location) and the stock a&b powders will be good for months and months. Maybe longer? Not totally sure exactly how long but it's a long while.