It's always best to have matching RAM - though you might find other RAM will appear to work, there can easily be small timing differencies that make for very intermittent "strange things" (caused by RAM errors).
So my motto has always been - why risk it. The (many - well in excess off a hundred) systems I've built over the years have at least been very stable.
Don't trust the siren voices that say - yes, these will work together OK - better play safe than sorry.
Also - always buy Class 1 branded RAM - that's from known makers who make the chips, put them on boards designed to JDEC standards, test them properly, and give a proper guarantee.
Personally my approach , if I couldn't find an exact match, would be to buy two new sticks and sell the old one. That may sound "over the top" - but that's what my experience tells me.