I wouldn't suggest running it forever with no fan since it is there for a reason, one or 2 days most likely won't hurt much. Any after market fan will not come with the bracket to hold it in place, it will just fit in the bracket you have. Here is a link to a fan that should work for you
I know it says that it is a 4 pin but it has ana dapter to make it 4 pin from a 3 pin. If you are going to the store to buy a fan just ask for an 80mm fan that connects to the motherboard with a 3 pin connector, they should be able to point you to the right fan. Make sure when you install the new fan you have it blowing in the case.