Those of us suggesting that you learn right handed aren't suggesting you play bass different than "the way you were meant to". Bass in not inherently a right or left handed instrument. Neither is bass guitar. It's similar to suggesting you go out and get yourself a left handed piano. You could likely get someone to make one for you, but there is no particular handedness advantage. Both your left and right hands have a lot to do on the double bass. Some would suggest you have an advantage if you are left handed on a double bass, because your left hand has to do all of the finger work. Others would suggest you have an advantage as a right handed person, because that hand is (largely) responsible for rhythm, articulation, and the sound you are producing. There is absolutely no benefit, with the exception of someone who experiences lost mobility/strength/digits on their left hand, for anyone to play it in either direction.
Likewise, if you play a similar instrument such as bass guitar "left handed" and feel that the learning curve will be easier when you switch, that's not entirely the case. The left hand and right hand technique (regardless to which one is doing what at this point) on double bass is quite different than it is on electric bass. The scale length is completely different, and even if you have no intention of learning to use a bow, the pizz attack is not the same. As someone who did make a switch from two very similar instruments (french horn-trumpet) with very similar fingering that involved using the opposite hand, it is not that difficult to overcome. At first it might feel a little alien, but if you stick with it it can become just as natural as your other hand. During that time you are learning things like finger/hand shapes, intonation, attack with your pizz/bowing hand etc. anyway. You aren't going to be able to hit the ground running if you find a double bass that is the same handedness as your electric anyway. There will be a learning curve, so why not use that time to make your life significantly easier?
What will be significantly easier? You will have the option of shopping for almost every double bass on the planet, instead of a statistically insignificant amount of left handed instruments you will most likely have to drive, fly, or have shipped to you to get anywhere near them. You will not have to pay a premium to purchase the instrument, or take a gamble on a custom commission that will take months if not years depending on the waiting list of that maker to get around to it. You will have a significantly easier market to sell to when you get around to selling it, and the entire market to shop from if you decide to upgrade. You will have teachers who feel comfortable teaching you, instead of trying to wrap their head around everything being backwards. You will have the option of playing in orchestras and string sections, even if that doesn't seem like something you are interested in right now. Ride a horse, don't go unicorn hunting.
Why spend the extra money, and seek out a builder that might not even make your short list when the other option is you don't have to? For a first instrument, there is a chance that the OP hates double bass and only sticks with it for a month. Why make it next to impossible to sell it again after that?
I am aware that there are a lot of lefties in the world. I am one of them. And I still play a normal double bass. I have nothing against left handed people, and nothing against people who already play the double bass that way for whatever reason they so desire, have pointed out it might be the right option for someone with an injury/chronic pain, and even suggested Upton as a potential source for such an instrument if you really want/need to go down that path. I am simply suggesting that it makes more sense to start a long and difficult journey without creating a needless disadvantage before you have even begun. If sissy kathy was looking for a double bass that wasn't left handed, sissy kathy could very well have an instrument in hand by now and already be learning.
I apologize if this seems strongly worded, extremely opinionated, or against left handed people. That is not my intention. I would very much like sissy kathy to not only succeed in finding an instrument, but to have a long and enjoyable double bass playing career. I am just suggesting that chopping off a foot before you start training for a marathon isn't the best way to get started.