A stability ball, just modify the exercise.
BTW you can order bosus online from amazon, or dickssportinggoods.com. Target might even sell them.
Unless you are not in the US you should be able to find one.
BOSU's are also at Linens 'n Things for $99, and you can find a 20% coupon in the Sunday paper almost every week.
I'm thinking about buying one. We use them in my Pilates classes for stepping, crunches, lunges plus lots more. We use the flat side up for pushups.