Hi !! Welcome to Sardinia and thank you for visiting !
Given the tight economy at the moment you may find it difficult to find a bike shop eager to take the risk. You may want to try to put and ad on
www.subito.it during the very last days of your trip,at the moment still the best and fastest place to sell/buy stuff.
You can also ask to local hotels/B&Bs and/or campings where you'll be planning to stay,they usually have a bike fleet for their customers to use.
I'll translate this for the other good guys on the forum,they may chime in for some tips. You should post some pics of your bikes,that would help a lot.
Enjoy Sardinia,paradise on earth.
Signori in poche parole per chi non masticasse inglese abbiamo due gentili turisti che alla fine del loro giro hanno bisogno di vendere le loro bici in zona Sassari o Alghero,cercano un negozio di bici disposto a comprargliele o gli va anche bene venderle ad un privato,nel caso fatevi sentire.