Dodd Camera in Cleveland develops 120 for me onsite, as does the MotoPhoto down the road.
Holgas are fun, they encourage you to break the rules, probably because the camera is such an indifferent piece of plastic. You don't mind pointing it at the sun or taking it out in the rain. The lens is a single element plastic thing, 65mm, which is a very wide angle for medium format. You need to get closer to the subject than the viewfinder would indicate. Rule of thumb: if the subject looks like it's exceeding the boundaries of the viewfinder, it'll be nicely framed in the negative. Be aware of parallax issues, the viewfinder is diagonally higher than the lens. There's light and focus fall-off from the center, which gives a nice vignetting effect. You need to wedge a couple of cardboard flaps from the film box underneath the spools before you start shooting, otherwise you'll get a loose wind.
Because the cruddy plastic lens is slow, you need to use a film no slower than ISO 400. If you want to shoot black and white and are having trouble finding a place that can process it, try Ilford XP2, which is a chromogenic black and white that is processed in the same way as color film, i.e., C-41. That way you may not have to have it sent out. A high-contrast film like T-Max 400 generally gives you darker results, which can sometimes be dramatic, sometimes just murky.
Look into getting a modified Holga from
www.holgamods.com. Randy fixes them up a bit (interior flocking to cut down light leaks, 6x6 mask, tripod socket, remote shutter release, bulb setting for time exposures, etc.), charges maybe just a little too much for the result, but they are more reliable when they've had the modifications.
The cheapness of TLRs is relative -- a couple hundred bucks for a Rolleicord or a Mamiya C-series is not bad compared to a couple thousand for something new, but it's way more than the $20+ for a Holga. Of course, with the TLRs you're getting real glass and professional quality images, while the Holga is an aleatory crap shoot. Lots of fun, though. In my experience, the Holga images that I think will look fantastic when shooting generally turn out execrable, while the ones I wasn't so thrilled about when taking them turn out surprisingly evocative.