When I resided the house with vinyl, I ordered all my material through Home Depot. Whatever they didn't stock, I simply custom-ordered it. (They can't stock everything, although, you might think they do!) Visit Builder's Edge WEB Site for product info. When you go to Home Depot, first check the Vinyl Siding Isle. I know they have Gable Vents there but they may not have your color style. If the latter is the case, go to the Building Material's Desk and ask for a Builder's Edge Brochure (or a brochure for whatever your store does business with. There may be MORE than 1 Supplier.)
As for what size vent you need, it will depend on a couple of factors. Since the roof is a 1/2 gable roof, ie. not a TRUE gable roof, AND if it gets direct sun (ie., faces South), you'd want a large vent. Also, the 'style' of the vent is a factor althought for a a 1/2 gable I'd get a rectangular vent. IF, on the other hand (your roof is like mine) where it faces North and gets only a few months of ANY sun, you could get away with a size smaller. The key to passive venting is to be sure your soffits are free and clear of any insulation AND you have either FULL-length soffit vents or a vent-hole between EVERY rafter. As you know, Nature will take care of 'pushing' out the hot air.
What I suggest is you talk to the folks at the Depot for what you should get. Find a style and color first, and see if you can get the right size. Don't fret if in a WORST case you can't get the proper size that have to stop everything. I know of homes that have undersized vents that do fine. Again, the key is the soffits.
My best to ya and hope this helps.
Jay J -Moderator
PS: I have 2- 18" x 24" gable vents for a 15' x 30' addition (if it means anything to ya ...) The pitch is 2-10.