What Are You Going To Be?
One source for ideas is the internet. Search with phrases like "Halloween costume for baby boy" and see what pops up. Search engines are meant to find the best page for the phrase you put in and it doesn't need to make sense for them to find it.
Another place to check is craft magazines. Some of these magazines have great ideas in them like making an old boy pirate costume into a renaissance boy costume; or use a hand me down costume from years ago for your new boy toddler costume. A big boy costume this year is next years Halloween costume for a younger boy. These magazines also have great ideas for DIY costumes out of things as simple as a sweat suit, some glue and felt. This is also where you can make a boy girl costume, such as a black jumpsuit with cut outs of bones. It makes a great skeleton and the materials can be found right around the house or at an arts and crafts store.
The cheapest place to find Halloween costume ideas is around the house. An old suit can be a boy dress up costume; some clothes with fringe sewn on the sleeves can be an Indian boy costume; an old leotard can be a boy dance costume, or if it is too chilly for that at the costume party, use it as a base and add a sweatshirt and some foil to make a space alien.
All it takes in the end for that perfect boy pirate costume or Larry boy costume is a little imagination and a little time. If you can't sew a Halloween costume for your toddler boy, use glue to put it together. If you aren't sure just who shark boy or lava girl are, do a quick internet search for them. If the boy's costume party is tomorrow and he is still sans costume, go to your closet and make your boy a dress up costume.
It all really comes down to what you are willing to build and what needs to be bought. You might not be able to make white face paint, but you certainly can make a skeleton's body. So think about the costume you want, and think of how you can make it yourself. If it seems improbably, go online and look. There are tons of ideas out there, not to mention commercial pages, and it never hurts to keep your options open.