I just won this Achievement about ten minutes ago, and ALL 15 RAMPS CAN BE FOUND ON THE VERY FIRST LEVEL (Auburn Pines Suburb) while trick-or-treating.
The level is divided into two halves: before and after you help the school principal, Ms. Putterpam (near the park), who won't let you enter the second half of the level until you get the ingredients for her pies to save gym class.
The first 10 ramps can be located in the first half of the level. Some of them are tricky to find, so count them down as you come across them. But there are 10 of them there, and you should not proceed to the second half of the level until you have found them all, to make this Achievement easier to get.
And the final 5 ramps are located in the second half of the level, and are all really easy to locate, as once you locate one, the rest follow in quick succession.
Totaling all 15 ramps, and the Achievement.