I bought a Warner & Swasey #3 turret lathe about a month or so ago. It has a master collet very similar in style to yours, but mine is slightly smaller. I have no desire to part with my collet anyway, but I thought I'd chime in to share information for whatever it might be worth.
I bought my entire lathe for a mere $300. It pretty decent shape and near 100% complete I might add. Including the collet and collet retainer ring. So I was very lucky to have obtained such a complete machine.
In any case, I looked around to see if I could find another master collet for it. Mainly just out of curiosity. What I discovered is that they are very hard to come by and very expensive when found. New they range from $750 to over $1000, just for the master collet. And that's if you're lucky to find someone who actually has one in stock. There may be places were you can pick them up used, but considering the prices for a new one I imagine even used ones will most likely be asking quit a bit.
Here's a link to my thread where I have pictures and measurement of my collet.
In any case, what I was going to suggest is that your best bet might be to look around for an entire lathe that has a collet it in. As I say I got my entire lathe for $300. I got it at HGR Industrial Supplies in Euclid Ohio. But it's my understanding that there are similar outlets nationwide.
The bottom line is that you might be able to get the collet by buying an entire lathe for less than you might otherwise pay for the collet alone. Plus you don't even need to worry about the condition of the rest of the lathe. You might be able to get your money back by just taking the collet out of it and reselling it with no collet.
Anyway, just a suggestion.
In fact, at the time I bought my lathe they actually had two of them almost identical for $300 each. I almost bought the second one just for spare parts. In fact, I would have done that if I would have had the resources at the time to pick up two lathes.
While I'm mentioning this, they also had a W&S #4 lathe for sale also for only $300. I was tempted to pick it up as well. Mainly for the 12" 4-jaw chuck that came with it. It would have been worth buying the whole lathe just to get the chuck.
Anyway, that's just a suggestion. Keep an eye out for any cheap junk lathes that have the collet you want. It might be cheaper to grab the whole lathe than to find a lone collet.
Just as a final note. Based on the design of these master collets I think I would make my own before I would pay something like $750 for one. I'm sure I could make one for a lot less than that.
Good luck in your search. Maybe you'll find one for $25 that someone has laying around and simply has no use for. Even though they cost a lot to buy, it might also be difficult to find someone who actually needs one. So they might be seen as fairly "worthless" on the market as a whole.
So best wishes in your search.