Hum, Alfa miniature is somewhat in my reach of competence...
If we exclude large scale, unique piece by modelling masters like Carlo Brianza or Michele Conti, there are quite few classic racing Alfas in large scale.
There are only two classic, pre-war Alfas in the 'common' ranges, the old Burago 8C2300, either in Monza or Spider Touring versions. Not quite as accurate as Franklin Mints, but so much cheaper!.
If you don't stick too much to the exact 1/18 scale, however, you may be happy to find that the Italian Revival / Casadio makes for over 30 years an even more detailed Tipo B (aka P3), built or in kit, in three versions: GP car, hillclimb with twin rear wheels or targa florio with a spare wheel. The classic model has a die-cast body, but there's also a cheaper all-plastic kit, and the scale is 1/20. There's as well a Tipo 159 in the range.
List and prices here - usual disclaimer. - just search the net for a dealer next to your place
Worth an eye, but in scale 1/24, is the 8C2900B Touring Spider by Franklin Mint, not imported in Europe. Very nice model. CMC is, as far as I know, the trade mark for the part of the Franklin Mint range imported in Europe, which doesn't include the Alfa.
eBay is a plausible source for one. Nominal price was $105+shipping, but I got mine in the 75-80 range+fees.
As far as I know, the very only scale model of the P1 was a limited serie, 1/43, by Paolo Tron back in the 80s.