TamaTalk is a pretty great little Tamagotchi forum. It has sections for help with various Tamagotchi and other virtual-pet related problems, group hatchings, growth charts, and more. It also has smaller sections for other types of virtual pets, too. It's one of the more active Tamagotchi forums that I've found.
Another Tamagotchi-based forum, this one isn't quite as active as TamaTalk. It does, however, contain many unique growth charts, some made specifically for the site that can't be found anywhere else. It's very helpful in that respect.
With lots of information and pictures about the majority of Tamagotchi releases, this is a very helpful site. It's user edited, so you may want to cross-check information you find here, depending on what you want to use it for.
This site has lots of information on vintage Tamagotchi releases, tons of logs and pictures, and even has information on other physical virtual pets such as Aibo and Furbies. It doesn't actually seem to have been updated in a while, but what's there is pretty helpful.
Provides basic information and instructions for the majority of Vintage and Connection/Plus era Tamagotchis and many other types of virtual pets, lots of pictures, and a few more in-depth guides.
Places to Buy
Japan You Want
This is a great place for buying Tamagotchis and Tamagotchi products. It's an off-shoot of their Japan You Want eBay page. I've always had great experiences here; they send orders out quickly and sometimes even include little extras with orders, like stickers. They only make you pay a single shipping fee, even if you buy several items at once; I've never paid more than $10 for shipping.
Another great place for finding Tamagotchis, though the quality of the product and timeliness of the order are going to depend a lot on just who you choose to order from.
My third choice for buying Tamagotchis, not as much variety as the other stores, and what's here ois generally more expensive than the others as well.