Normally get these when on special at the supermarket, but I was running low and nobody had a special currently active. Thanks to Google, I found this offer direct from Sodastream. I bought 3 x 6 packs for the free shipping so each bottle works out to $4.68 each.
Pepsi varieties include Pepsi Max, Regular Pepsi, Mirinda, Mountain Dew and 7Up.
Individual bottles also on special at $5.46 (Reg $7.00)
Other specials on their site are as follows.
Soda Press $7.76 (Reg $9.95)
Classics (Tonic, Lemon Lime Bitters, Summer Lemon, Raspberry, Ginger Ale, Cream Soda, Lemon Lime, Ginger Beer)
6 Pack $26.10 (Reg $29.70), Single bottles $5.35 (Reg $5.95)
Zeros (Orange Mango, Lemonade, Pink Grapefruit, Cranberry Raspberry, Lime)
6 Pack $35.70 (Reg $41.70), Single bottles $6.25 (Reg $6.95)
I bought some syrups direct from Sodastream once, but everything was super short-dated (weeks), so don't buy lots if you're planning to stock up. Their customer service didn't care, and had the attitude of 'they are consumable now so that's not our problem'.