I use liquid Kyolic... you can get it in some supermarkets, it's a small bottle but it's pretty concentrated (I put 5 or 6 drops on a frozen cube)... it costs about $15 IIRC.
I went to Trader Joes's and found a frozen pack of little squares for $2. Then just put it in a small squeeze bottle and fill with RO/DI and keep in the Fridg.