In "The Gift of the Magi", I need a quote that can be used to link it to a theme of "material items bring joy".
This is a rather interesting question, because actually, the gifts that Jim and Dell sacrifice so much for, whilst at first are greeted with joy, are then greeted with unhappiness at first when they realise what both have done. For example, consider how Dell responds to the gift of the combs:
And then an ecstatic scream of joy; and then, alas! a quick feminine change to hysterical tears and wails, necessitating the immediate employment of all the comforting powers of the lord of the flat.
So, it is a little difficult to find a quote proving that material gifts bring joy, except if you are thinking about how material gifts give more joy to the giver than to the one receiving the gift. Consider how Dell is presented when she gives Jim his gift:
Jim had not yet seen his beautiful present. She held it out to him eagerly upon her open palm. The dull precious metal seemed to flash with a reflection of her bright and ardent spirit.
Note how her gift mirrors her "spirit" and the joy she has of giving him this gift.
So, to conclude, I suggest you think about how giving gifts gives more joy to the one giving them than the person receiving the gifts. Really, this is a story about self-sacrifice and the love that drives Jim and Dell ironically to give what is dearest to both of them up for the other.