Pretty much, any 6 volt non-rechargeable battery with the required plug, is going to do the job.
NOTE: Fitting a rechargeable battery is pointless, because the 5170 motherboard does not contain battery charging circuitry.
Original battery
Below is an original battery.
Click on picture for larger view
It is 6 volts, Lithium (not the same as Lithium-Ion), and IBM part number 8286121
IBM don't sell the battery anymore, but there are many battery suppliers on the Internet who can supply a replacement.
Many of those list their replacement under part number 8286121
The pictured battery has a resistor on top. The replacement battery does not need to have a resistor.
Four 1.5 volt AA cells
Using the battery holder pictured below, four standard 1.5 volt AA cells can be used.
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A search of eBay using battery holder aa should find this battery holder.
3.6 volt Lithium
Some people use a 3.6 volt Lithium battery.
With a 3.6 volt battery fitted, some 5170 motherboards may keep the CMOS settings but lose time. (technical info)
A particular type of 3.6 volt battery is pictured below.
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A search of eBay using battery tadiran 3.6 should find this battery.
