Spacemacs recommends using source code pro as the default font, infact this is the font set in .spacemacs file. If the font is not availale it is substituted for some other alternative.
To find the font in use do C-u C-x = on my mac it was using Menlo font. DejavuSansMono looks better than Source code Pro on ubuntu and I switched to it in my ubuntu system.
A list of top programming fonts can be found at Top programming fonts
Installation on a ubuntu system is easy, copy all the <fontnames>.ttf to ~/.fonts folder and then update cache using fc-cahce -f.
To get the list of installed fonts use fc-list
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