Try www.getzclub.com
Plenty of tips & Tricks. 
Beware of snake oils though, total waste of money 
Check ot www.mycarforum.com,
look for topic Genie 47's hall of shame -snake oils....this lists
all the common bullshit found in the markets.
For your car, definitely do a CAI, not open pod though. The GETZ's
stock air intake sucks in hot air from the engine bay, doing a CAI
would help. U can replace the stock air filter from
Green-Filters....found to be better than K&N. Think u can also
find info on this filter in mycarfroum
U may try your luck with a piggy-back chip for a little more power,
upgrade the stock exhaust to a free'er breathing one.
Can upgrade to sport alloys but I personally think u should stay
with 14 inch rims as 15 inch rims will affect the pick-up..since
the engine does not make much torque to start with.
Anyways mods that are worth your $$$ are CAI, a performance filter
(I do not think an open pod is advisable here), a better exhaust,
piggyback chip(if available for Hyundai) and steel braided brake
hoses for better braking. These should give you better efficiency
and noticible improvement in performance 
Short of turbo-charging and supercharging, it is difficult to
easily extract copious amounts of power from the 1.3L SOHC
lump.