They are considered a seasonal item and so sometimes hard to find during the off season. We went through this before also. I did think Walmart carried them most of the time though...
We have made use of the non clip on variety. I am just using one of the smaller desk fans we had underneath our 75g. It fits with our odd design and actually gives more air flow. Also on top of the 75g, we have two of those really small desk fans that blow down into the lights. Everyone's set ups are a little different, so you just have to find what works for you... good luck.
