Since your not planting a bog or something large, you might just consider washed horticultural grade sand. I get it at a nursery and it comes in about a 1 gallon bag. Also, you might consider pummice you can usually find it at large nursereys.
Play sand is often collected from rivers, and if it's near agriculture thre is a good chance that there is fertilizer in the sand from run off.
I'm not a big fan of planting the bottom of a terrarium. There are a couple of drawbacks in doing this. One is, you are limited to the kinds of plants you put in it, unless you are willing do dig a plant out that requires dormancy. Also, if a plant becomes infected with a disease or pest, you have to dig it out before if infects the rest of your plants. If you have them in individual pots you can simply take the pot out. Many times in undrained setups, the planting media becomes stagnant or minerals build up and you must replace the entire media. Again in pots you can just replace an individual plant's media.