You can probably call your local iriver office and ask them to sell you a pair of replacement earbud, and I am almost 100% sure it will cost you more than what you are willing to pay for earbud of such sound quality. If SQ isn't any concern of your, just visit a local electronic store and pick up a pair of Sennheiser or Sony.
If SQ does mean something to you, than order a pair of Yuin PK3 from
Head-Direct.com for $50 (or $80 for a pair of the better sounding PK2). It might not look much like a high class earbud, but don't let the look fools you. It worths every cent that you pay for. If you don't trust me, just google 'Yuin PK3' and you will find dozen of reviews.