If you don't trust your calibration fluid, try making your own and see how it compares to the store-bought stuff. From Randy Holmes-Farley:
[Speaking about 35ppt sea water]: This concentration roughly corresponds to ¼ cup (73.1 g) of Morton's Iodized Salt dissolved into two liters (2000 g) of water (giving very slightly more than 2 L of total volume).
(Full article
here.)
The readings of any refractometer can change over time. Theoretically, as long as you recalibrate every so often with a known good solution, you should always be getting solid readings. Unless of course the refractometer is damaged and changes readings between calibrations.
If you're worried about your refractometer being damaged or broken, I'd first confirm that your solution is good (try the home made solution as mentioned above, or take it to a LFS and ask them to help with some of their water). Then, after you get your hands on some good calibration fluid and you've successfully calibrated your refractometer to 35ppt, recheck it every week. If you see the value changing, that could indicate a problem: refractometers shouldn't be falling out of calibration weekly. If, however, your readings are stable over the next few weeks, I'd continue to use it. Just be sure to re-check the calibration every few weeks.
As an aside, most refractometers recommend calibrating to zero with fresh water, but I'm not a huge fan of that method. It doesn't really matter if the refractometer is accurate at 0 ppt because you're not using it to measure 0 ppt water. You're using it to measure 35 ppt water.