The other answers answer your specific question of buying interruptor brakes. Interruptors are fine, but they only work when you're riding the bars and you can't shift on the bars. You should really fix the main problem which is comfort and reach when riding the hoods, which is where most people like to have their hands when road biking. It's both a safety and comfort issue because you need to be able to steer, brake and shift.
First I'd like to answer the pre-question as to why you can't ride the hoods, both from a woman's perspective and then from an adaptive sports (arthritis) perspective.
Women have smaller hands then men while ... surprise-surprise... brake levers are designed by men for male hands and as a result, tend to be too large. Even women's bikes tend to use male components because we're not a large enough market (yet) to have our own. And you have the added issue of arthritis.
1) Reach: Almost all road bike brake levers can be designed to reduce the "reach" necessary to grab the brakes. This video shows how to adjust the reach on a typical road bike, but you can find more by just googling road bike brake reach adjustment:
This thread on bikeforum notes that the Sora STI's do have a brake reach adjustment:
The Sora STI levers on my Giant SCR have adjustable reach.If you look on top of the hoods where your hands go there is a hole, and inside that hole is the slotted head of a screw-turn this and it moves the lever closer to the bars-it a bit of a crude adjustment as the screw is in effect an adjustable backstop,but it works.
In the absence of an adjustment screw (which is the case with some older brakes), you can glue a small wedge into the gap created when you pull the brake levers. This prevents the brake lever from returning out all the way (and is effectively all that the adjustment screw does).
For more information, there is also a related question: How do you adjust the set-up of road bike brakes for women with small hands
2) Brake sensitivity: You can also tune and adjust the (presumably caliper) brakes themselves so that they are more sensitive / require less pull to actuate. You'll need to do this when you adjust the brake reach. If you tune it finely enough, you should be able to balance reach and good brake sensitivity even with a smaller range of motion on the brake lever.
In addition, if you switched to a mechanical disc brake (which would depend on frame/fork compatibility), you could further increase brake sensitivity. For example, I can do a forward somersault (not a good thing) with just one finger pulling just the slightest distance with disc brakes. In fact, I find I have to detune my disc brakes slightly now while I get used to them.
3) Position and Comfort: You can also adjust the position of the lever so that it rides up higher or lower on the drops. Depending on the ergonomics of your hand/wrist, this might give you better reach. Some padding on the hoods might improve comfort at the expense of reach.
4) Body: If it's the size of the hood itself that is too large, you might be able to find a smaller brifter/brake lever itself.
5) Handlebars: have you considered changing handlebars? Even if you kept the drop style you could find slightly wider or narrower options, or a different radius on the drop curve.
Finally, if after all the adjustments, the drop style just isn't working for you, have you considered changing handlebars to a non-drop style? If you can't ride the drops or hoods, you can't shift. So you might want to seriously consider changing out the handlebars.
6) Adaptive solutions: You might want to talk to your sports physiologist, physical therapist, occupational therapist, or adaptive sports center about what your other handlebar and brake/shifter options might be. They might recommend straight bars, a slightly curved bar, modified aerobars or something else. Which ever solution you have, you want to be able to steer, brake, and shift (in that order of importance). Since interruptors only do the first two, I'd still strongly suggest not going for that stop-gap solution and exploring further.