Some ikea stuff is nice because the design is modern as opposed to the horrid dominican style, but be carefull, don't buy anything that is made of timber (or appears to be), it will be done in 6 or 7 months.
plaza lama is hopeless for anything at all, anything I've ever bought there has broke within months, from baby stuff to ovens, I'd never by anything there that I wanted to last.
the best stores around with best prices can be found around Oriental, there are several decent stores on autopiste San isidro. Infact there is a place that sells high quality furniture that is solid in Coral Mall (on auto-San-isidro).
further up at the cross roads for George De-G and Auto-p-San-isi there is a huge store with a big variety, mainly Dom stuff but worth a look.
beside Banco Pop on Auto-p-San-isidro upstairs there is a floor dedicated to furniture stores, also worth a look.
infact there are just loads on Auto-piste-San-isidro, not even considering the rest of oriental.