You should do an open-circuit voltage test of your battery after a good long drive - at least 30 minutes at highway speeds. If it reads 12.4V or better then it's fine. If it reads below 12.4V but above 12.2V then it's likely marginal. If it's lower then I would bring the car in to a garage and have the charging system and battery tested.
If you just need a replacement battery, your car probably needs a Douglas battery. Douglas is the official supplier for the original as well as the replacement BMW battery. The battery they will sell for your car is actually rated higher in performance than the one they make for BMW. Try their webpage for import car batteries:
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I have a 540i and it has the same engine as your car and probably uses the same battery - yes, it's a rectangular battery. I believe mine was the 49-675 (BCI Group 49). I bought mine from a Douglas Battery reseller for about USD 100. If you call the toll-free Douglas number (1-800-368-4527) the customer service agent will look up the closest dealers in your area.
You can experiment with other makes of batteries but why would you want to experiment if you can buy a guaranteed correct or better battery at the same price? It's not trivial to be stranded by a dead battery.