You can source some of the "O" rings from engineering suppliers, the hardest one to find an exact equivalent for is the main body "O" ring, there are usually three or four of them in a gun, however it depends on the range of "O" rings that your local supplier stocks. I have ordered "O" rings in, only to find that they do not quite fit, being slightly too thick in the rubber cross-section. Smaller overall diameter "O" rings can be stretched slightly to fit which have the correct rubber cross-section, but it does not always work out. Buying the Mares repair kits is a good way to go as they should include the piston's rear rubber cone seal as well as the correct number and sizes of "O" rings. Plus you know that the gun will hold pressure straight away, provided you assemble it correctly, rather than have a "suck it and see" approach with substitute "O" rings. Smaller "O" rings like the trigger transmission pin seal are easily replaced with rings from other suppliers. The Mares "O" rings are metric sizes, something to watch out for if only Imperial sizes are available to you, however some of these will also fit if they are the same size equivalent. The rubber hardness is just the standard durometer for "O" rings, nothing special on the guns that you are working on. It is worth measuring the new kit "O" rings and noting their sizes for future reference, that way you can source additional "O" rings at a later date. These days "O" ring sizes are defined by the inner diameter of the rings and the rubber cross-section diameter. There used to hundreds, if not thousands, of different sizes listed in the old "O" ring catalogues where a number of other measurements were quoted, such as outer diameter, but things seem to have been simplified now.
Oil to use is motorcycle fork oil, it comes in 500 mil squeeze bottles from auto spares stores and has different viscosity grades, SAE 10 is what is recommended by Mares, so that is what I use.
As you have taken the guns apart before you know what to expect, cleanliness of the workbench is paramount and you need plenty of lint free, clean rags on hand to wipe parts clean. I use wooden toothpicks to lift the "O" rings out of the grooves, you will bust plenty of toothpicks, but they will not scratch the "O" ring seats which is very important, especially as they are all moulded plastic grooves on modern Mares guns. I also use grease to lubricate things, just a thin flim, from a big tub of Valvoline "X-All" grease.
Patience is a virtue when rebuilding these guns, hurried gun repairs often have you repeating the exercise, so a slow, methodical approach is recommended. Keep notes as you dismantle the guns so that you know how to put them together again, unless you have a very good memory and can remember where everything goes.
