You cant buy a tamale in the store. To get a tamale, you need Power Shroom + Power Shroom + Golden Poo

But there are other ways you can get a watermelon.
Cherries + Cherries + Mango Poo
Strawberry + Strawberry + Tamale
Strawberry + Strawberry + Acorn Squash
Mango Poo + Quartz + Small Topaz

http://pakkapets.wikia.com/wiki/Item_%26_Recipe_List
This site was a big help for me.