old horse needs new shoes

76 MR 250 going to be used for trail riding. Wanting to be ready for anything (mud, dirt, snow, sand...etc.) and needing to replace tires. Wanting to change to knobby's as bike should never touch asphalt. any suggestion as to where or what I should buy? Also, how do you read tire size now? Wanting a 3.00 X 21 and 4.50 X 18. But I cant find tire sizes like that anywhere. any chart or advise would be appreciated.

Thanks! I was cruising a site and found Metzlers that were listing 100/100-21 and 80/100-21for the front and I did find a 120/100-18.I will have to investigate further now that I have an idea of what to look for.