I would give your babies a little dab of Nutri-Cal on your finger probably 3-4 times per day. I like to scrape it on the roof of their mouths, as my babies have never liked the taste of it (although some dogs do! lol). Just a tiny dab should do it.
Natural Balance can be purchased at most Petco stores. Petco carries a few really high quality brands, but they also carry the lower quality stuff, too. Petsmart doesn't carry any of the really good brands unfortunately. At my Petco, the price of the Natural Balance Sweet Potato & Venison went up...$37.99 for a 15lb bag, and $57.99 for the 28lb bag (I think that was the price of the 28lb bag). It went up about $5-$7 since the last time I bought it. lol I thought I would mention that just so you have an idea of what you will be paying.
Also, if you visit the Natural Balance food website and sign up for their emails, you will occasionally receive coupons in your email.