...but I can't find a source code for SDP4 on the internet.
As discussed in the question How do “Deep space” corrections in SGP4 account for the Sun's and Moon's gravity? and according to Wikipedia:
Current code libraries have merged SGP4 and SDP4 algorithms into a single codebase handling the range of orbital periods which are usually referred to generically as SGP4.
You should check the documentation of the implementation of SGP4 that you are using carefully to find out if it includes SDP4 or not. Chances are that it does, but to be sure, it's best to confirm it.
Can I just use SGP4 for geostationary orbit? if yes, what is the consideration?
If your SGP4 already contains the SDP4, then there will be an internal "switch" that turns on the deep space corrections from SDP4 when the period is 225 minutes or longer, so there will be nothing to worry about. It will happen internally and you'll never know.
If you somehow have an old SGP4 that does not have the deep space corrections, then the effect will be quite small for short periods of time around the epoch of the TLE. Most GEO satellites will have new TLEs issued regularly, so it won't make much difference.
If you are using TLEs for times very far away from their epoch, and using an old SGP4, then there will be small errors.
If you can't tell if your SGP4 has the correction, then you can follow up with a new question, something like "How can I tell if..."