You can usually get them at a craft store but I found these while looking for something else.
kits (http://www.snowdomes.com/makeyerown.html) and more from oriental trading (http://orientaltrading.com/application?namespace=browse&origin=searchMain.jsp&event=link.itemDetails&demandPrefix=12&sku=48/3388&mode=Searching&erec=0&D=snow%2Bglobes&Ntt=snow%2Bglobes&Ntk=all&Dx=mode%252bmatchallpartial&Ntx=mode%252bmatchallpartial&N=0&sd=COLOR-YOUR-OWN+SNOWMAN+SNOW+GLOBES). I would totally shop around before ordering though.
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kits (http://www.snowdomes.com/makeyerown.html) and more from oriental trading (http://orientaltrading.com/application?namespace=browse&origin=searchMain.jsp&event=link.itemDetails&demandPrefix=12&sku=48/3388&mode=Searching&erec=0&D=snow%2Bglobes&Ntt=snow%2Bglobes&Ntk=all&Dx=mode%252bmatchallpartial&Ntx=mode%252bmatchallpartial&N=0&sd=COLOR-YOUR-OWN+SNOWMAN+SNOW+GLOBES). I would totally shop around before ordering though.