Ah that link wasn't the one I intended to post, though there's some good information on spotting cuts there. I'll see if I can dig up the correct link.
It shouldn't be too hard to get your hands on pure Ketamine, especially if you said you're purchasing it in amounts too large to throw away - get your hands on some unopened vials instead of buying powder/crystal

Use your imagination about where to get some, everything is available if you know where to look!
One good way to purify it at least a little would be to wash the Ketamine with various solvents, picking up impurities and cuts in the process. Try dissolving it in water, ethanol, and other solvents that Ketamine will dissolve in, filtering it through coffee filters/paper towels/paper filters and then evaporating the solution - each time it should remove any impurities that aren't soluble in that solvent.
MSM itself isn't going to cause any real problems IVed that I know of, but as I said the concern is more that all you know is that it's cut with MSM, and not how many other substances it could potentially be cut with.
There's always recrystallisation, but you might lose some Ketamine in the process, so a few washes is an easier solution, and can at least get out some common impurities.