IMVU Black Market: http://www.imvumafias.org/Catalog.html
IMVU Gray Market: http://www.imvumafias.org/community/buysell/
You can also find these markets by googling "IMVU Mafias".
The Gray Market is known for having cheap competitive prices as there are tons of people on there fighting to give out the best prices. On the underground Gray Market, there is everything from free items to items you have to pay for. They accept payment with paypal, credit card, debit card, echeck, western union, ucash, IMVU credits, and file trades.