I was in Rome last month, and discovered a nice shop:
Nair Hobby di F.Muri
Circonv.Ostiense 287. 00154 Rome
Phone: 06 5743900
It is not in the centre, but it's worth a visit. Also be aware it is open evenings only. No web for this one (as far as I know)
In Barcelona there are several shops, for example:
- Hobby Galan
 Psje. dels Garrofers, 12 TF.(93)2001139 - Fax.(93)2021568 - 08041 BARCELONA (SPAIN) 
You may visit their web at 
www.hobbygalan.com- Jordi Rubio 
Av. Gaudi 56. 08025 Barcelona
Phone: 93 433 45 53 Fax: 93 435 78 79 E-mail: 
[email protected], website: 
www.jordirubio.com