I went to Hasbro.com and Hasbrotoyshop.com and there where only three transformers products. So how do you? thanks -Rattraprules98
Is it something recent or something older? Older you'd need to try Ebay or the Junkion Exchange. Newer you can certainly try contacting Hasbro, but be advised, they might ask you to send in the whole product for an exchange "with the same product or another of equivalent value" - i.e. you might send in your Banzaitron without one of his swords but get back a Defender Optimus Prime. Generally, probably better to go back to the stores in those cases. All others - ask around, see what you can find in the Junkion Exchange.
I once bought a replacement part from BBTS for my MP Megatron (that I bought elsewhere) as he came incomplete MISB. Depending on what you're looking, they might have what you need if they got returns.