Welcome to the hobby! Leave your wallet at the door, and we'll get started.
The Chaski machinist forum has a section specifically for 3-in-1 machines-
The Home Machinist! - Index page
Also in case you didn't already know, many full-size accessories will not fit on benchtop machines. Littlemachineshop is one online seller that specializes in the smaller machines and parts and accessories for them-
LittleMachineShop.com - tooling, parts, and accessories for bench top machinists
For general machine-tool supplies, I usually look around at Enco first- www.use-enco.com
(site is down at the moment for me)
Shars is another place that sells a lot of China stuff- shars.com
(Shars also sells some items even cheaper on eBay under the seller name discount_machine )
I have a mill and lathe, separate benchtop machines from Grizzly. Great fun but you can sink a lot of money into accessories, even if you stick to buying the cheapo China-made stuff. It's nothing to spend $100 for a handful of mill bits, taps, tool blanks or some odd measuring doohickey.