Absolutely get your preapproval first, then hook up with a buyer agent. In Berkshire County, we have VERY few foreclosures, but the process is not very easy and IT TAKES TIME. If you are trying to buy your first property to take advantage of the tax credit, time is NOT on your side, as you really should have something fully under contract within the next couple of weeks to have ANY hope of closing before the deadline! Home inspectors, appraisers, and attorneys are getting backed up already, and the unexpected always comes up at the worst possible times. Delays WILL be a problem and some buyers will miss out on their tax credit because they can't close before November 30th.
Foreclosures have complicated steps that the lenders follow in processing offers. Its not like you are negotiating with one person - you have to jump through many corporate hoops. It may take weeks, not days, to get any reponse to your offer. Additionally, if someone comes in willing to pay cash, and you need to finance the purchase, even if you are preapproved and your offer is higher, the cash buyer may end up getting the property that you wanted.
Additionally, foreclosures may have problems of which you will be completely unaware. It is unlikely that there will be a seller disclosure of defects - a corporation across the country has no knowledge of the property's unseen "issues". This is the risk you assume when you buy a foreclosed property. It will be an "as is" sale, meaning you will have no recourse if there are problems you discover after the sale. That can get expensive very quickly.
If you need to take advantage of the tax credit, I have to recommend at this late date you should avoid foreclosures. There are a number of really great properties on the MLS, and super buyer agents who can help you every step of the way to help get you closed by November 30th. Did you know that the seller normally pays the buyer agent's fee, not you? Take the free, experienced professional advice that is available to you!!!!!!!
Good luck in finding your first property!