I’ve cut down the 1/4 inch pads from MLD with no problem, using a large drywall square to mark the line and scissors or utility knife to cut. No problem. Sometimes they only list the 1/8 inch version on the website. If so, call them and they may actually have the 1/4 inch or be able to tell you when the 1/4 in is available. I’ve used both the 1/8 and 1/4 and much prefer the 1/4 if R value is highly desired. I don’t carry my 1/8 inch much any more, but it does have its place.
RE: the comment above from Chris K about the Ridgerest in 25 inch width; good idea…But… A few weeks ago I spoke with Thermarest / Cascade Designs seeking a 25 inch Ridgerest and they confirmed (and they were surprised) that the 25 inch pad is not currently listed on their internal product list and they do not know when or if it will be available. All the Ridgerest pads are made in USA, so that response was a surprise to me (and to the customer service rep).