Chestnuts are not for sale, you will have to look for them yourself in autumn. They are very suitable for play or to serve as weighted material. If you dry them immediately on a towel near the central heating or another warm spot, they can be used for a long time. After a while they will stop shining, but you can use them for years without getting mouldy.
For suggestions for games see Playing with chestnuts.
If you have gathered chestnuts in autumn, hot-water bottle covers are useful to put them in. You remove the hot-water bottle and put in the chestnuts. The cover can then be used to massage your child with or to hide under. For the latter game it is more fun if you have more than one hot-water bottle cover. Of course you can fill them with different substances, like beans, rice, or macaroni. When you use smaller substances like rice, I advise you to put the rice in a plastic bag or several plastic bags first, before putting it in the hot water bottle cover.