There are a total of 40 riders which you have to get to max level,which is level 5. This involves you opening booster packs which give you cards for riders and equipment,you have to get 10 of the same cards for each rider to obtain max level. All the booster packs contain random cards so there is no sure fire way to get this other than purchasing the packs and hoping for the best.
The best (and quickest) method for this is once you've finished the campaign replay the final stage(27) and just fail, all 6 bulls will take around 3 minutes resulting in 600k per time. I used this in conjunction with the open 250 booster packs achievement and was buying bronze packs. You get 1 rider card for bronze(20k) 2 for silver(40k) and 3 for gold(60k).
Once you get a rider to max level then just sell any duplicates,along with any equipment you don't want to buy more packs.
I did this around 18-20 times before I got the relevant cards but it may take less/more due to the randomness.
This solution is just the quickest way to grind money for the cards.