When you're in the main lobby of the Draw Something app, scroll down all the way, and you will see a button called shop. Upon clicking on the button, you'll be brought into the Bomb shop where you'll be able to stock up on bombs before your next match.
The prices of the bombs are as follows,
1) 400 coins for 10 x Bombs
2) 720 coins for 20 x Bombs
3) 1,600 coins for 50 x Bombs
If you have insufficient coins to buy bombs, you can click on the '+' symbol on the top left hand corner beside your coin counter, you'll be brought into the coin shop. Here, you'll be able to purchase coins with real life money:
1) $1.99 for 400 coins
2) $4.99 for 1200 coins
3) $9.99 for 3,000 coins
4) $24.99 for 10,000 coins