You should first ask yourself: what is my idea of a spa? Then go from there. A whirlpool tub seems essential, appropriate lighting fixtures (or candles , some nice towels and last but not least bath products (sea salt, mineral mud, etc.)
Any of the home improvement/renovation stores should have brochures on this subject. Check out Ikea catalogue for ideas as well.
My type of SPA should be a simplified SPA, i.e. no whirpool tub.
I guess what I want is "aromatherapy".
I found "Fruits & Passion" in Bayshore Shopping Mall. www.fruits-passion.com
I saw a very nice set of spa products from Costco today, for $59.99, you get a complete set, from bath salt to essential oils, from foot scrub to eye pillow, hand lotion, body lotion, you name it. It comes in 2 fragrances too: honey almond and menthol & vanilla.