First it's only a plastic part because of the telescope you bought. Usually a diagonal is all metal.
Many companies sell diagonals they come ion 3 sizes .965, 1.25, and 2 inch sizes. Measure the hole in the back of the telescope and buy the correct size.
Here are two retailers:
http://stores.ebay.com/Telescope-Warehouse
http://agenaastro.com/optical-accessories/diagonals-prisms.html?SID=rbfrqab9jjkmrk7bd6ci0sg534
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