Might I suggest you google 'Alghero Tourist Office' as their website will probably have a list of accommodation, and then contact the hotel websites directly to get some idea of price. If you then cross reference your hotels with Trip Advisor you might find some input on which hotels are likely to suit you and which are better avoided!

Be aware that 15 August - which falls on a Saturday this year - is a major holiday in Italy (Ferragosto) and the whole country will be on the move, so make sure that you have somewhere booked well in advance and have also booked somewhere to eat for this weekend!!