I got mine at Amazon.com for $3.60 usd. It is 30x and has a light on it. The cost included shipping. I tried Walmart first, and I agree that they have nothing better than a 2x or maybe 4x magnifyier. Ebay would be a good choice as well. Radio Shack sells a 60x-100x mini microscope for about $12.00 usd that works, but you have to adjust the focus on it and the hole you look thru is so small that it is probably more trouble than it is worth.
I use a 30x, it allows you to focus in close, and get a good picture of where the majority of your trichomes are in terms of development... Higher scopes generally get too close, and give you a crap time trying to get a good view of a patch of trichomes.