If you seriously want to box you need acess to a ring, sparring partners and above all things a real trainer:

http://www.boxinggyms.com

http://www.boxinggyms.com/addressbook.htm

http://www.boxinggyms.com/findgoodgym.htm

Alternatively I recommend asking around in local gyms and sports shops, they often get given numbers from clubs just incase some one like you asks for a gym :P

comprehensive list for Canada, by province associations.

Provincial Association - About Us - Boxing Canada

list for Ontario, by club and contacts.

Club Directory

anyone has links they want to add, for your country or area, please go ahead.

thanks.

Not solely boxing gyms but Gym Visit is very comprehensive Google Gym Visit