I think your quilt idea is wonderful and one your friend will treasure. I do not know of any die-cut hearts, but I think it the result would be much nicer if you did it yourself. 40 hearts sounds like alot, but it could be made easier if you used a template (a simple one cut out of cardboard) and sharp scissors. You could even cut several out at once by layering fabrics. Your fabric choice would be unlimited and the quality of fabric would be much nicer than if you bought something pre-cut. You could even have a little dinner with some friends and have them help cut.