I think EBay would be your best bet , I see plenty of carbs on there all the time ....
but even then , I would go thru it to make sure nothing is clogged from sitting , and you would usually at least have to rejet it for your elevation ..... but I have bought a few nice carbs from EBay and got them for a decent price (decent would be in the $100-$150 price range , average would be in the $300 + range)
Any of the 450R carbs will work , and the only real difference is the jetting and needle shape , (you would change that anyway for your elevation) , and the float bowl has a drain screw on the X when the R usually does not , so you can get a new OEM X bowl and be good , but basically any 450R carb will work as they are more common ....
Unfortunately , lately , everything bike related has skyrocketed in price , so finding one for a decent price is going to be tough right now , last time I bought a carb on EBay (about 2 years ago) I got it for $150 (05 450R carb) it was really clean , now the same carb is $300 -$500
https://www.ebay.com/itm/Honda-crf450r- ... Sw4UVgKHsE
Unfortunately , this is the only 450X carb I found , you can find off brand repops for cheap , but I do not know what the quality or longevity would be for $150 new , you could try one , but I would not guarantee fitment or quality of the parts on it , it may work fine , or it may have been a waste of $150 .... but this one below , even though it is a genuine 450X carb , it is about what new would cost from the dealer , so you would be just as well off getting a new one from Honda (shown in pic below) for the price of the one in the following link
https://www.ebay.com/itm/CRF450x-Carb-C ... SwUnJcj~Gh
New Carb Cost
