Max - yes, you can buy a pedal that simply activates the overdrive in your amp, but only if the amp has a footswitch input. Check the amp manual or the manufacturer's web site. If you are happy with the distortion sound of your amp, that is the cheapest way to go.
As far as actual distortion pedals out there, they do not control your amp switching, they modify your guitar signal to create various distortion and overdrive sounds. Depending on what musical styles you like, there are certain pedals that would be more suitable to your taste. Some good brands to check out are Boss, of course, MXR, Line 6, Zoom and Digitech. Behringer have a line of pedals that are very similar to Boss, but for much less money.
Since you are new to pedals/effects boxes, you might want to check out a multi-effects box or pedal, by the same manufacturers listed above. They offer a wide variety of sounds in one box. For example, you can pick up boxes like the the Line 6 Pocket Pod or Digitech RP-50 for less than $150, maybe even less on eBay.
Good luck!