You have already found Romanarmy.com and this has probably led you to RAT (
www.romanarmytalk.com). If not, you will find there an enormous number of discussions on all aspects of the Roman army, some of which may be relevant to your project. As you have noted, online discussions of themselves are not much use as sources but they may contain links or references that can be cited.
There is really no substitute for books or articles in academic journals and I wonder whether your local library could help here. In the UK, there is the Inter-library Loan Service whereby a library can borrow a book from another library and then lend it on to you. It can also get photocopies of journal articles, usually from the British Library. Is there a similar facility in Finland?
Unpublished theses can be found on the British Library EThOS website (
http://ethos.bl.uk) and many of these can be downloaded free. If you search for 'Roman' or 'Roman army', you will find a number of theses, some of which may be relevant. If your researches turn up a journal article that interests you, you may find it on JSTOR. If so, some articles are available for purchase. You do have to pay a few dollars but, once your payment has gone through, you can download the article immediately.
I hope that some of this is helpful and that I have not been teaching my grandmother to suck eggs, as we say here!