You will love nestabilites. I prefer combining a large scallop with a small shape or a small scallop with the large shape that fits inside. That way there is a more pleasing border around the shape. That means you are better off to buy the set of four like this http://spellbinderspaperarts.com/classic-ovals.html
when you can. From my limited experience it is good to go to www.ritzcamera.com
in their scrapbooking section where you can search "nestabilities." Also, if you want to buy one at a time, www.cutathome.com
has a 40% off coupon on one item each month. Hobby Lobby has a limited supply, and using a 40% off coupon is a good price.