How to make my work publishable if my supervisor isn’t any help?
The foremost thing you need is a mentor familiar with the subject to guide you through the process of paper writing and in particular provide feedback to you. Unless you are very good at it, writing a mathematical paper without any feedback is a bad idea, as it’s very difficult to estimate yourself whether your writing is understandable to others. Also, scientific writing is an art of its own that you probably have not mastered yet.

As you already guessed, the canonical choice for such a mentor is your supervisor, but as he is not an option, I would consider the following (in roughly this order):


other members (PhD students, postdocs, etc.) of your supervisor’s group who have experience in paper writing;
visiting scientists or collaborators of your supervisor;
other scientists from your department, with whom you are on friendly terms;
scientists from other universities doing related work;
look for a mentor on your department’s bulletin board or similar.


To avoid quarrels with potential mentors and because it’s ethical, you should clarify the following:


You are not good at estimating the publishability of your work. It may very well be that it turns out to be unpublishable.
The essentials of the situation between you and your supervisor.
How you will handle authorship: Just mentoring your paper writing does not qualify for authorship by common authorship standards, but it’s not unlikely that your mentor will, e.g., find some extension or improvement of your work that is worthy of authorship. Authorship may be a relevant incentive for mentors. For more information, see authorship.


Together with a mentor it should be much easier to decide on the publishability of your work and what journal is suitable for it.

Finally, some further caveats:


Depending on how much your supervisor laid the intellectual foundations for your reasearch, e.g., by giving you detailed ideas on what you should research and how to do it, he may have reasonable authorship claims. Unless he verifiably abandons these claims, this would lead to all sorts of trouble.
If you are worried about your work being stolen, you can publish the current state as a preprint, e.g., on the ArXiv. Be aware that there are journals that do not allow for such prior publication, but as far as I know, this is not a problem in mathematics.