your GT610 will not have any external power connectors on it, that goes for gt630 as well. This is because those are not "Performance" graphics cards, they are somewhat basic graphics cards. the PCI-E slot that the graphics card plugs into provides (generally) about 75Watts of power to the card, which is more than enough to run something low end. I think most of the GTX750s do not require extra connectors, the 750ti 1gb version mostly do not require external power. the GTX750ti 2gb need external 6 pin power. So it depends on what you ordered, but eitherway, assuming age of your PC (your 6 series GPU), then you would likely have a 6pin connector on your PSU, although it is just not in use right now, you have to look at wiring output from PSU and find it. If this is a prebuilt system, then I would say your chances are slim of having that 6pin connector. Eitherway, a decent PSU to power a GTX750ti would run you only around 30-40$.
Another thing, i'm not sure if you are talking about gtx750 or gtx750ti... as for today, you really have to TRY and get a 2gb vram gpu, anything lower than that is just not up to the task for today. I assume you live outside of USA that's why you bought 750 new?