I personally use http://posterburner.com really cheap and usually pretty quick with good quality. I always wonder what's going through their heads when they see the stuff I want printed.

https://www.catprint.com/home

i have heard much praise for catprint from artists who sell prints regularly. They always talk about how quality the prints are and the good customer service.

Seconding Catprint! I used them recently. Their response time was great via email, you can ask for free samples, and will provide a free hard proof [that one might have a size limit, not sure], and they send you an email to confirm when something is shipping to you. Also, no minimum orders!

I've honestly found that the best outcomes are usually using a local printing shop; unlike online services, you can talk with real people and look over proof-prints in house, and sometimes talk about things like color balance, brightness, etc.

You might be best served by a google search for your local area for print shops (the same kind that service businesses, print flyers, etc.)

Also: Sign printing shops can often do printing on plastic or even aluminum stock for SURPRISINGLY cheap.

Unfortunately local print shops are either severely over priced, or can't get an image to print right for anything, and want to charge you full price for every failed attempt. (Wish I were joking.)

I make all my prints at home with Red River paper, couldn't be happier with the quality of their papers!

For me I use either Walgreens (for quick or a few pecies) or for other work white house custom color printing https://www.whcc.com especially for there matalic prints. Granted most of my experience has been with photograph with them and not computer graphics.

 sixthleafclover and I use our own printer for printing prints and posters. We have an Epson SureColor P800 professional photo printer that uses 9 colors. It can achieve full color spectrum, matching RGB quality on your monitor. It even go up to 17-inch wide posters (standard size 17 by 22 inches). We don't print on matte, gloss, or semi-gloss papers as those will either make the colors look too dull or add too much shine that will throw off the contrast. We use a specialty resin-coated paper called luster. It combines the best of matte paper (no reflection or glare) and gloss (vibrancy of color) and none of the drawbacks of either type of paper.

We do offer a printing service to other artists looking for cheap short-run printing needs for conventions. We have quite a few artists coming to us for their printing needs. The service is quite economical if you're in the US as the shipping cost is much cheaper.

http://www.sixthleafclover.com/printing/

My issue is that my sizes for artworks vary, and none of them are 'standard' sizes for posters. I really should switch to standard poster sizes I suppose.

We can trim them for you. Trimming the small prints (letter size) is free. Trimming the posters do cost extra since we have to bring out the cutting board, the guide, and the xacto knife.

I used to use Walmart, they had superb quality for real cheap. It does take some at home editing with contrasts to get it to look right sometimes, but more often than not they were super good.

I've since bought my own printer and make my own prints at home, but if you're looking for anything larger than 8.5x11 i guess that isn't really much of an option unless you plan on shelling out a decent chunk of cash for a good printer with more than 6 tanks

I've heard some good things from my friends about http://www.indynexus.com/

It's the same folk who run Ringtail Cafe, and help make lots of elements of FurReality.

you ask a bit too late. i printed until a few months ago :D but my printer died and a new one is waaaaay
to expensive for so less customers i had, so i stopped. Only my A3 printer is left ^^ and a lot of photo paper XD
i thought it would be nice if people could get printed their furry stuff without thinking what the one on the other
side thinks about it, no matter if sfw or nsfw.
but it seemed nobody wanted it and then i stopped by business with the end of my printer ^^