Perhaps you are thinking of the ball? Or there is a bubble blowing thing (it looks like a polka dot flask with a lever)
In any case, you have to wait for an item you want to come into stock in the shop.
Each day the shop restocks itself - new items when your Tama wakes up, also at 3pm and 7pm - so keep checking what's for sale.
Some people also re-stock the shop by changing their Tama's clock to 2.59pm, wait one minute for it to change to 3pm, then enter the shop to see what new stock is there.
I don't know how this might affect the way your Tama evolves 'though - changing the time a lot tends to slow down the growth process.
Best, etc.
TamaMum