018 is a standard size - 3/4" ID, 7/8" OD, 1/16" CS. Material's not very important, as long as they're reasonably soft (like standard nitrile/buna o-rings). You can probably find a box with 10 o-rings of each size from tiny up to about 2" diameter in Halfords for less than SP probably want for two measly o-rings
To get the front one in, remove the barrel front and put it in from the front. A pointed stick comes in handy to coax them in, as long as it's not sharp enough to damage them.