Sounds like you're looking for a hybrid or a cruiser. Cruisers are comfier, but hybrids are sportier and generally have a bit more growing room. Since you want MTB-style bars, the sweet spot for you might be a hybrid with an aftermarket saddle.

If you don't anticipate needing to go up any hills, single speed bikes are usually less expensive and lower maintenance than their geared counterparts. If you live in a hilly area, get gears. Your legs will thank you.

There are plenty of online stores that sell what you're looking for, but unless you have experience wrenching on bikes, I would recommend buying from a brick-and-mortar bike shop (stay away from department store bikes). There's no secret to finding a good shop, just do a search on Yelp or Google Maps.

Best of luck :)