I recommend that prints are mounted and framed rather than just framed. I think it just focusses the image better. My prints are sized at 18 x 12 inches (457 x 304 mm) and will fit standard size 24 x 18 inch (609 x 457 mm) frame with mount or bespoke ones, which aren't as costly as you think (for example in the UK Easyframe). Amazon stock many standard size frames for this size.
Image shown has a 40mm mount.
If you have any other specific sizes you wish to order please contact me and I will try and accommodate this and will give you a price. Please bear in mind that this will take longer than the standard delivery times.