I don't know where you are, but I would first try the place where you got the unit. According to the online users manual, the spa has a 1-year warranty. If you are in that period, contact Intex Service Center (number in the manual and on their website) for warranty repairs. If out of warranty, Intex probably sells the replacement parts so again try calling them for the part or to identify an authorized dealer near you.
Failing all that, try eBay or other online sources.
Lastly, I'm guessing you've already taken the control unit apart to determine the heating element is bad. The element itself may not be replaceable by itself and you may have to buy the next higher subassembly. Not likely, but very possible. Also, this is a unit with electricity and water in close proximity. For safety, be very very careful after repairs reconnecting the unit.