Hi gcardullo83,
Congrats for yor first post!
You can get abbots as spare parts on eBay or directly from Cundco.de (crazy shipping to the US) but you have to bear in mind that aboots are only useful on monasteries and on gardens. The latter are not present on old artwork tiles, and that is the case of BB5.
A workaround to play with abbots would be to get some extra plain meeples from Meeplesource.com and tweak them with a rubberband, for example. You could use the vineyards from Hills & Sheep as gardens as a plan B and test the concept.
To enjoy abbots fully you could get a new edition base game so you have the whole pack (gardens and abbots) for a decent price or even opt for a BB6 although the contents is not the same as BB5.
Mixing stuff from both editions comes with the issue that the gray figures were replaced by pink figures, so this is another issue to solve.
So your question was a simple one but it involves many decisions...
Don't know if this helped you or made your life miserable with so many choices to make.
If you need more help with any of the options, do no hesitate to ask.
Cheers!
PS: The German forum created conversions of the garden tiles for the old edition... This is another option if you feel like creating the garden tiles for yourself... but this is another story.