1) Look for colleges and universities in your area. Go online and search for some school of higher education near the place were you live. Very often music departments of these universities offer classes and degrees in organ performance. Consequently, they also must have some organs at their disposal. While the large organs in concert halls might be difficult to get access to, the small practice organs are perfect in your case. Introduce yourself to the local organ professors and ask for possibility to use their instruments for practice.
2) Look for churches in your area. Because most of the organs are built for churches, see if you can find some which are close to where you live. Don't be afraid to approach local clergy and ask them for possibility to practice on their organs. Many churches are empty in between of services and you should be in nobody's way. Of course, it is wise to ask for a constant practice time which will be available for you.
3) Offer volunteering once a week in exchange for organ practice. Sometimes clergy might want something in return, so if you are just starting to play the organ, a little of volunteering can be a good idea. You see, very often churches have organist shortage these days and if you are competent enough to play a service by yourself, this can be a great practical training opportunity to get more experience. Who knows, maybe part-time organist position might become handy with some extra income when you retire.
Note that I am not suggesting you should always volunteer to play for services, this advice is only for people who are at the beginning of their organist career and want to get more experience along with some practice opportunities.
4) Get to know local organists. Usually organists are very supportive for people who are interested in learning to play their instrument. Introduce yourself to the local organ community, attend their recitals and talk to them afterwards. Use the power of social media to find organists from your area and interact with them.
Don't forget to congratulate them on the success of their recital and ask questions like how long did it take them to learn a particular piece or how did they become interested in organ playing. Once the conversation is active, tell them that you are also interested in learning to play the organ, the only problem being getting access to an instrument for practice. After such introduction, your organists might offer some ideas or even their instruments. You can even ask for paid lessons, if the person seems good to you.
Use one or all of the above tips today and perhaps tomorrow you can practice on the real organ.
You can also download my FREE video guide: "How to Master Any Organ Composition" in which I will show you my EXACT steps, techniques, and methods that I use to practice, learn and master any piece of organ music.