30 mm is really like the industry standard....so most of the ...US Optics ...nightforce...leupold..springfield...some places make 35mm available...like US Optics but its not the norm.....just gives you some more MOA adjustments.
Most quality scope makers offer a 30mm series of scopes. The defacto standard is 1" (25.4 mm). To list a "few" of the available 30mm scopes.
Leupold - Several models, some Mk4's, some others.
Burris - Black Diamond, Euro Diamond
Nikon - Tactical Series, Monarch Gold
Bushnell (B&L) - Elite 3200 Euro 3-9x50mm
Swarovski, Zeiss, US Optics, Super Sniper, & springfield armory all offer 30mm also.
Most high end scope makers have some sort of 30mm tube scope