Sounds like you've just taken recent ownership. Don't jump in too fast too hard all at once. You may need to invest in more important fundamental components. Make sure the vehicle is first operating as it should before immediately purchasing & installing a FGS. As previously mentioned, use the "B" shift in the meantime. It may be good enough for the time being.
But if you really want one, post your request in the WTB thread or check the For Sale thread. There's usually a few available.
Let us know what hemisphere you reside in, also. A Selling member may be relatively close by.