Yeah you are one of the many caught out by that. You need to do your research or ask the dealer when ordering if they have been superceded which they have been in this case. At some point suzukis changed from black to blue and the mid 80's DR250 and DR200 have the same shape/size boots as the earlier RM/PE and all they did was change the colour to blue. Suzuki says these are the same boot, just different colour and they still fit, so the older RM/PE numbers are changed to the later blue fork boots and this is what is on the world wide data bases. I guess there theory is blue boots are better than no boots. When you ring the dealer and ask for fork boots for 77 RM250, they will look up the part number, and it will update to a newer one and then it will say 'available' or 'not available' or something like that. The key is to actually ask them is the part superceded and in the case of colour specific parts, make sure you are clued up on suzukis colour codes before they order you in a part that is not really the colour you wanted.
Yeah Cemoto or Daystar are the closest ones you will get in aftermarket ones that look right on the older bikes that are teh right diameter and have a lower amount of ribs.
For Daystar you would want series 58 or maybe 70 and in Cemote you would probably want 2321 or 2322 depending on model/travel etc. There are a couple others too.
Check out the Daystar website and Cemoto website for more details/options. Australian wholesaler for Cemoto is Pro Accessories and they sell the Cemoto boots and you can check their site too.