I'll try to answer it the way I understand it. The difference types of size ratings are not necessarily because one is for offroad and one is for street. It's just that one measurement is being given in inches and the other is being given in metric. The link Auke supplied (and there are many others if you search) is simply giving the conversion between metric and what they are calling "Alpha", whatever that is. I call it inches.
So your 3:00/21 is the same as a 90/90x21. Your 4.60x18 is equivalent to a 120/90x18. If you get a smaller size tire it will screw up your handling (for one). If you get a larger tire, it may have clearance problems or be too wide for your rim. Now, different tire manufacturers are going to offers different style tread patterns in those sizes for various uses. You now need to do some searching on the internet, or go to the shops, and see what suits what you are looking for. You will unlikely find a pure street tire for a 21 inch rim. You can find some good 50/50 tires or something similar.
Here's an example for Metzeler Tires. I use the Sahara 3 on my bike which gets more street use than offroad.
http://www.metzelermoto.com/web/catalog/moto/moto_catalogoDett.page?categoria=/catalog/moto/enduro_street&vehicleType=MOTO&menu_item=/products/catalog/enduro_street
Look at the various tires, see what type of use they are recommending and then see if they are available in your size. if it is listed in inches or metric it makes no difference as long as they are the equivalent size ( I listed above). Then, check other manufacturers and see what you find. Once you find the tire you want, the hard part begins;
finding a source in Thailand.
Hope this answers what you were asking. Good Luck :thumbup: