You are right, Avianca Taca is connected to United, but I think UA earns miles in a TA/AV LifeMiles account but TA/AV doesn't earn miles on UA right now.

AV earns miles on IB but TA does not, they have supposedly merged but they definitely need to do some housekeeping.

Continental is due to be added soon so CO miles can be earnt in LifeMiles but likewise there doesn't seem to be any plan to earn TA miles in CO.

LifeMiles has 3 US credit cards (No linky because it is a dynamic site) they earn between 10,000 and 20,000 miles. The 10,000 miles card is a secured card so one would imagine it would be pretty impossible to not get approved.

LifeMiles is cool in that you can use copay for rewards. ORD (I guessed) to San Juan is 22,000 return, so if you only had 10,000 miles say from the secured credit card it would let you pay cash for the remainder.

If you get the 20K card then it would cost an additional $29 plus taxes
15K card = additional $126 plus taxes
10K card = additional $160 plus taxes

Taxes are $74 so say if you got the secured 10K card, it would cost you an additional $234

As FlyingBoat says though, it is tight, you would have to get the CC then get the miles posted and still hope there is award availability