You don't have to get the Jamaican black castor oil. Regular castor oil, the golden kind, works fine for most. And it runs you like $8-10 at the health food store. I used the NOW brand (actually cost me $7) and mix a tbsp of castor oil or more with my deep treats. I don't use it alone typically as it is so thick and gummy. (I can't attest to it making my hair any thicker, however, because I already have thick hair.) Just a note--you do need to cut castor oil with a thinner one to help it spread as it's not a true "oil". My mother has been using a mixture of 50/50 castor oil and coconut oil on the edges of her scalp to help them grow a bit fuller and it's really working. Also helpful is the advice from user aqua_lily. She says to take some castor oil and mix it with coconut, just dip your fingers into the mix so that they are slightly wet, then tap the scalp gently and give yourself a super gentle massage, letting the sparse coating of oil absorb into the scalp. (Not too much oil unless you want to wash your hair. Just use enough to make your fingertips shine.) So this method has certainly worked for her in terms of hair growth as she's already gone past waist length hair.
3A - C, HP, ME, HD. (Coarse, High Porosity, Medium Elasticity, High Density.)

CG since Nov. 2012

Poos: SM Moisture Retention + Yucca Baobab, TJ's Tea Tree Tingle
Condish: TJ 'sTea Tree Tingle*, SM Moisture Retention* + Curl & Shine + Yucca Baobab, Yes to Blueberries
Stylers: KCKT*, SM Curl Enhancing Smoothie* + Curl & Style Milk*, KCCC*, FSG*, CJ Pattern Pusha, Curl Keeper
Sealers: Jojoba* or Grapeseed* oil

* = HG