Almost any nursery will have them. Big Box stores may or may not, but you can usually count on nurseries to have them this time of year.

GGG

Are there any particular varieties that you've found are easy to care for?

There are many. I would suggest that you go to any of the nurseries in the Woodstock area:

Autumn Hill
http://www.autumnhillnursery.com/

Ladyslipper Rare Plant Nursery
7418 Hickory Flat Highway 140
Woodstock, GA 30188
770-345-2998

Perennial Grace Nursery
14395 Birmingham Highway
Alpharetta, GA 30004
(770)569-5448

Twin Branch Nursery
1169 Wiley Bridge Road
Woodstock, GA 30188
770-926-8566

These should all be places that can do right by you. They carry plants that do well in our climate. Big Box stores tend to carry whatever they can get. Sometimes they get great plants that do really well in our climate, sometimes not. They do not always have the widest variety of choice in what they carry - it is only whatever their list of suppliers have in stock that they can choose from.
Nurseries can get whatever they need, and whatever will do well for the customer.
If you watch, you see different customers at each type of business. Often the Big Box stores know that their plants will be purchased by "weekend gardeners" and so even perennials are going to be annuals for some folks who lack that "green thumb". Nurseries tend to cater to folks who garden more frequently or are looking for specific things.

GGG

This site has some good hardy geraniums available by mail order.

Here is a link that might be useful: Geraniums

I'm pretty sure Perennial Grace is closed down but if you are in that area Scottsdale farm right down the road might have them. bobby

Dunwoody Nature Center is selling Geranium 'Rozanne' in their spring sale. That is a very popular one.

Here is a link that might be useful: Sale link - you have to pre-order by Apr 3