Do they sell Xbox Live membership cards at Walgreens?

Yes, they should sell them at the gigantic game card rack or card in general for at least 20 dollar cards.

There are a few sites you can buy discounted Xbox Gift cards and Live Gold Membership cards from:

GK4.me,

CrazyGameTime.com,

MMOGA.com,

G2Play.com.

12 Months cards for as low as $39.99, etc. They all accept PayPal so you don't have to disclose your credit card information. Have fun.