Carles,
If your aim is to model the car, it doesn't matter too much whether the pics are actually of any of the 4 MM cars, or even from other races those cars entered. You will find more on the web typing "8C2900" on any resarch tool. You'll of course find also many diffrently bodied cars, but there are shots of surviving cars as well.
Back in 1938, after the Mille Miglia, the same MM Spiders also raced at the Spa 24 Hours (with extra fog lamps in the front wings) and in some hill climbs (by memory Pontedecimo-Giovi, Colli Torinesi, Stelvio). In 1939, one at least ran at the Luxembourg GP on June 4th. Those were Alfa Corse entries. Later, cars were sold to privateers. Swiss drivers owned several, notably Daetwyler. Phill Hill also raced 412030 in the USearly 50s.
For sure, THE source about those cars is Simon Moore's book "The Immortal 2.9". This book has one down side: it's out of print and estimated `£350.
When looking at surviving cars, take note that there are three:
412030 is the only one with its original body, but some detailling do not match anymore the 1938 form (very minor)
412031 is an original MM chassis on which a body has been rebuilt after 412030's. While not original, it's by very little the best IMHO
And then 412021. Replica body, but not a copy of the existing survivor. Overall impressive, but not exacly the right thing. You find complete doc of it at:
http://www.symbolicm...00/8c2900a.htmlCompare that one with the original, and you'll realize how much Touring was absolutely great. But you'll have a lot of different angles which would help for modelling.
This is 412031, as far as I can tell:

There are some existing models of it. The most common is Brumm and is absolutely wrong. The front end looks almost as it ought to, but the tail really comes from another car!
Carlo Brianza did possibly the best 1/43, John Day also made a kit in the 70s and Plumbies Inter (Danhausen) produced quite a nice one in the early 80s.
Recently, somebody made a 1/10 detailled model on sale for only $4000 (I would take a dozen, please)
If you make a little serie of models, let me know,since I also collect Alfa miniatures.