^^^To add to Rhett's sentiments I suggest World Market as a popular place for that. (just discovered) They have a huge selection of classic retro style soft drinks and gourmet style. Generally the genuine sarsaparilla is a little different than root beer and quite interesting. Cream soda is always a plus. Wherever you can find it, get it.
Forgive my ranting as I was *quite* the soda connoisseur up until recently here. I appreciate the chance to share if you will indulge me...
Before I continue, health concerns:
More often nowadays I tend to drink unsweetened kettle brewed black tea to help me control the addiciton. The soda addiction has serious health consequences down the road. If I do drink soda, I tend to want the stuff with real cane sugar because
1) it is possible to be satiated and, you know, STOP drinking without fiending for more.
2) I notice HFCS soft drinks tend to keep one coming back for more, and more, and more.
3) I am wary of stones forming in the gallbladder and a number of other things (adult onset diabetes, increased chance for osteoporosis, digestive imbalance, interferes with calcium absorption, etc.)
Being in my late 20's I'd like to remain rather more healthy into old age than less.
You heavy soda drinkers have been warned.
1) I have a Soda Stream a client simply just gave to me (doesn't want her kids drinking soda) and I go to Staples and other places to get the syrups. So I am enjoying making my own sodas currently. Boxes of purified water (I use Smart Water from Costco personally), refrigeration and any soda syrup of your choice. Hard to go wrong there.
2) Whether cramming for a test or surfing the web, Sevvy is a nice late night stop for big gulps with coke or doctor pepper with either cherry lemon or vanilla flavoring added. But I swear their junk food is lethal. BLT potato chips for the win.
3) Delis in general where I'm at tend to like to supply "Izze" which is starting to catch on with my taste buds. Pomegranate namely. Give it all a try. Less sweet, more flavor.
4) Jarritos are a favorite though I don't see my favorite flavor 'Jamaica Jamaica' anymore.
SafeWay usually has some where I'm at. Mom and pop Mexican restaurants usually have Jarritos too.
5) Raley's notably has a "gourmet" soft drinks section. Not all of it retro but there is quite a variety, 15-20 brands at least. Teddy's is pretty good, so are "vintage" coke and pepsi. Jackson hole. Bubble up. Sioux City. XXX. Ramun'e (Japanese). Oh there's so many I can't remember them all right now. Either it's in their deli section or their soda aisle. Go treat yourself.
Occasionally I see Rock'n'Rye Faygo at Raley's, which I liked. They used to have this blueberry soda I'd drink with lager or other beers and actually it set pretty nicely.
6) You ever tried stuff from Henry Weinhard's? *Excellent* stuff. Worth buying a box (or two) of anything you an get your hands on.
I DO caution you, though, be careful how you refrigerate it AND how and where you open it. They can sometimes surprise you like you can open it and it sits there but about a minute later the foam starts creeping out. Don't take quick "swigs" either because you might just agitate the foam that way too.
7) Kemper's is definitely good but at times a little too saturated even for cane sugar... at least some flavors are. Worth a try, though.
8) Vernor's ginger ale is a distinct flavor for special occasions. Hell any ginger ale. Can't quite handle ginger beer though. Tends to burn as it sits in my digestive tract. Certainly try a hand at it.
9) I *really* like the stuff at Trader Joes. Virgil's root beer, vintage cola, Orangina, their seasonal lemon italian soda (Mmm yeah! DEFINITELY!), their grapefruit soda, and their italian blood orange soda (which you can get year round). Every other week I get blood orange fron TJ's--great for your italian food night. Which leads me to...
10) You can also get Italian Blood Orange soda occasionally from Costco (seasonally), and hit or miss from Grocery Outlet, Big Lots, and occasionally at grocery stores in their deli. Try them all, you'll be glad you did! The aforementioned World Market has it all the time as well.
I hope that helps you search for the next great carbonated flavor.