I had a liquor in Curacao named Ponche Kuba.  I want to buy it in the
Denver Colorado area.  If it isn't available what other liquor can be
substituted for it instead.

If you like Ponche Kuba, you might want to try the dutch liqueur
"Advocaat." It has a similar flavor.

"Advocaat:

A Dutch liqueur consisting of brandy, egg yolk, and sugar. It is
derived from an alcoholic drink which Dutch colonist in South America
made from the yellowish pulp of the abacate fruit and the name,
already evolved into avocado among Portuguese colonist in Brazil,
Became Advocaat in Dutch. In the Netherlands there are many different 
brands of Advocaat available on the market, some flavored with fruit
and vanilla. Instead of using this liqueur in the stereotypical
snowball, try wetting a glass with brandy and then pouring in the
advocaat:The brandy prevents it sticking to the edges of the glass.

Approx:(17% abv) 34º proof

 bol's Advocaat
 
 Cooyman's Advocaat
 
 De kuyper Advocaat
 
 Keeling's Advocaat
 
 Verpoorten Advocaat
 
 Warninks Advocaat"

http://membres.lycos.fr/cocktails2k/history.htm

My favorite Advocaat is made by Verpoorten:

http://www.bevmo.com/productinfo.asp?area=spirits&find_spec=verpoorten&catname=

From this article, it appears that Bols is gearing up to introduce
Ponche Kuba to the United States, but I haven't seen any evidence that
this has happened yet:

http://www.totalpp.co.uk/page.cfm/Action=Press/PressID=126/t=m

Notice that the article linked above compares Ponche Kuba to Advocaat:

"Ponche Kuba is a drink which originates from the sunny Caribbean
island. It is a creamy delicacy which contains eggs, rum and brandy
amongst other ingredients. 'Many inhabitants of Curaçao make it
themselves at home, just like we make advocaat,' says Piet van
Leijenhorst, head of technological services at Bols in Zoetermeer,
Netherlands."