Even if you did have a reputable source of good bacteria that you could put in your filter you'd still, in all likelihood, and depending on your stock, have some teething problems regarding a show of ammonia. I took mature media from my old sump which was running a 180g, put it in my new filter for my new 360g and added the fish thinking I was done. Far far from it. I got a scary ammonia spike within 24 hrs.
Water changes, usage of prime and careful monitoring got me through it with no casualties. If I remember rightly I think it took a week for my BB to catch up.
What I believed happened in my case was that the bacteria from my old sump which I added to my 360 filter was only a % of the bacteria which was needed to sustain my stock. The other %, which wasn't added to my new set up, was in my 180g tank, on decor, glass etc.
So although I thought I was doing good, in actual fact there was a severe shortfall in BB numbers, which led to my ammonia spike.