I use Krita, which is free and open-source and is available on OSX, Windows, Linux and (I believe) soon-to-be Android? Krita is good for painting and drawing, if you want a photo manipulation program, try GIMP, it is also free and open-source and supported by the same 3 operating systems. I use both programs to do digital image works.
I don't really use it for drawing, but Pixelmator is a pretty affordable Photoshop alternative. I say Photoshop because it seems to support a lot of brushes, and has lot of other photoshopping and vector art features, but may not be as good as a dedicated drawing or painting app. I don't know for sure because I haven't really branched out lately. There is also a Pro version now, which I haven't touched at all, but even that version still looks rather affordable.
if youre desperate to run sai on mac, try using wine! i use wine on linux, linux and mac are somewhat similar (both unix based) so i think it should be able to run something like winebottler. i think krita is a good alternative to sai though but i'm not sure how it runs on mac since i don't have one.
I literally had this exact problem when I got my macbook lol. I switched over to Clip Studio Paint after some fiddling around. You won't get the great blending/coloring that PTS offers, but this is the closest I can get without using Photoshop. Unfortunately, you just missed Clip Studio's sale so I think it's back up to maybe 40-50 usd. And CSP does support pen pressure on mac!