Compact cranks have smaller, compact bolt circle diameters (BCD) at 110 - which allows for smaller chainrings. They offer gearing options fairly similar to a triple without actually being one. They are currently very popular and new ones can range from as low as $75-100 all the way to several hundred. Going with a compact may require you to replace the bottom bracket as well, which will add to the cost.

Gear can be bought a lot of different places - but if you are looking for someone to do the work, I'd suggest a local bike shop. Finding just the right one can be like finding a good car mechanic - sometimes you have to talk to a few to find one that you trust and is reasonably priced.