Any online PC shop should sell case fans (generally 120mm size). You can search for fans that have filter covers which work pretty great, especially if you have pets. They reduce the maintenance of cleaning the interior of your PC dramatically. Just pop the filter out and wash it once every few weeks and youre good to go.

Being Doha though, you will still get quite a bit of dust inside and you should at least blow it out with a can of compressed air or use a paint brush to dust off your components / heatsinks every few months.

Theres nothing you can really do to completely eliminate dust getting inside your case in this country without going through some extreme measures.