Don't order refills online (unless you can get a great deal on the sodas you like).
Even a year ago, I had trouble finding some flavors in-store, but now it seems that SodaStreams are popping up almost everywhere! Though you can buy SodaStream SodaMix at Amazon.com, they are priced a few dollars higher than if you bought them at a local store that sells the syrups. Still, if it's a flavor that you love and it is hard to come by, it may be worth the extra cost. Just be careful about over ordering, as the syrups do have an expiration date.
Bed Bath and Beyond + coupons = savings!
Save those $3 off $15 and 20% off coupons that you receive in the mail or Sunday paper. If the syrup retails at BBB for $4.99, then 20% will save you a dollar. Bear in mind, you are able to stack the 20% off coupons, so if you have a few saved up, you can stock up and save more since you can use a 20% coupon on each item in the same transaction. Don't toss your expired BBB coupons either --- they'll still take them, so long as they were a mailer with your name on it (they do not honor expired coupons that come in the paper.) You can also exchange your CO2 canister there as well and use a coupon to offset the cost of approximately $15.
Le Gourmet Chef coupons
If you have a Le Gourmet Chef nearby, look for those coupons that print at the register that are Bounce Back bucks. Save the coupon for a future purchase (or trade your CO2 canister there too), but note the expiration date on them as it is usually a month or less to use.
Buying a SodaStream on the cheap
With the SodaStream popping up at a wider range of retailers, chances are that you can pick one up cheap by using store coupons or combining offers with free shipping at sites like JC Penneys (who offers the Fountain Jet for $80 every day), Target (online only for $79.99, though they do have them in their retail stores too), and Sears (for $89) Bed Bath and Beyond also sells SodaStream machines, and you can use the 20% coupon off to bring the price down. Pair your best coupon up with a site/store that sells a SodaStream and avoid paying full price.
Enter to win a SodaStream over at Turning the Clock Back's blog, whose contest runs through 5/18/2012. Good luck!