I can recommend
http://www.selectmotorracing.com/
It's reasonably priced & very flexible. It's run by a husband & wife team who are F1 fanatics.
It's also very easy to go independently. The local authoriries of Milan & Monza get together to provide brilliant public transport. There are regular trains from Milan to Monza & there's a free shuttle bus from Monza station to the circuit. On race day there is a free train from Milan Central Station to Lesmo & back. You can also get the metro & then a bus to the circuit or there are buses from outside Milan Central Station.
Nearer the time of the race
http://www.monzanet.it/eng/default.aspx publish a transport guide.
The circuit is open on the Thursday & you can get in if you've got a race ticket but I've never been asked for mine, I've just walked straight in. It's well worth a visit & you can get very close to the cars.
Some are more popular than others though!