There are 5 listings of Spences born 1826 plus or minus 10 years with mother's maiden surname Hen(d)ry in the statutory deaths index
Mary, 50, 1871, St Ninians
Elizabeth, 40, 1875, Glasgow St Rollox
James, 70, 1886, Brechin
Elizabeth, 66, 1886, Peterhead
Isabella, 71, 1902, Rescobie
At the moment the mother's maiden name is only in the deaths index from 1855 to 1910 and from 1974 onwards, so it's possible there might be others after 1910 - you'll just have to wait for that information to be added and then check again.
And of course the mother's maiden name is only added if it was known to the informant at the time of registration. So there could be more, but they would be a long and potentially expensive job to work through unless you were able to visit the Scotland's People Centre.
On the other hand, Isabella Spence seems to be living alone in 1841, (FreeCEN transcription
https://freecen1.freecen.org.uk/cgi/search.pl) so she may not have had many, or even any, more children, and almost certainly none after about 1825, unless they were all away visiting granny or someone on census day.
At that time a pensioner specifically meant someone who had served in the military and was receiving a military pension, so be wary of using the word pension.
If she was 'Independent' it means that she was not receiving money from the poor's fund, so she must either have had some capital and been living off the income from that, or she was receiving some sort of annuity. However any such annuity would have been arranged privately, not through any state agency, so there will be no readily accessible records.
There is a possibility that there might be something in the Register of Deeds but they are in the National Records of Scotland in Edinburgh and it could take some time to trawl through even the indexes - and not every year has been indexed.
Another outside chance is that Aberdeenshire Archives might have something on her.
Once upon a time there was a web site with transcriptions of all the gravestones in the old kirkyard in Banff, but I haven't been able to find it again just now.