Inner tubes are basically rubber balloons that will expand to fill the available space in a tire. As such there's plenty of latitude in tube sizing. You can see this by the fact that single tubes are rated for a size range ie. from 1.9" to 2.125" as was you original. That said, One size fits all works about as well for tubes as it does clothing.
Most bike shops stock 26" mountain tubes in 2 or 3 width ranges, to cover tires from 1-1/4" wide to 2-1/2 or so. Look on the oiutside of your tire for the width, and buy a tube of the appropriate range. If it's on the border of two, sizes I usually go smaller rather than larger.
The slime option is up to you, it might make sense if you've been plagued with lots of small punctures.
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