When I was in Spain a couple of years ago I bought tickets as and when at the ticket office. It seemed that there was a flat rate of 22 Euros for each AVE ride - or was that 22 Euros for the two of us, I can't remember now.
Strange, I have certainly had FIP tickets off DB in Surbiton before. When do you want to travel? Bearing in mind all long-distance services in Spain are compusory reservation it is not possible to book until the trains are released for sale (something like 3 months / 90 days IIRC). As you are going into France as well try Rail Europe , but I have never tried buying FIP over the phone from them, only in person.
I would not recommend waiting to buy tickets until you get out there as trains can (and often do) get fully booked and, if that is the case, then you will not be able to buy a ticket or even get on the train.
PS. If you only want to travel on local or suburban trains then they are not compulsory reservation (or very comfortable to go long distances in!) and you would be better off getting tickets locally. Did Valladolid to Santander and back to Palencia (300 miles) on a "Cercanias" unit a few years ago for the huge sum of 8 (FIP fare) and got a square backside for my trouble
FIPs can be problematic to obtain for travel in Spain partly due to the booking horizons. However I found http://www.rail-canterbury.co.uk/index.html very helpful in arranging tickets for me for a trip last year. They are happy to deal with queries over the phone, but will charge a modest booking fee.