Where can you buy jumbo covers?
I've found that if you don't have a current "standard" ironing board, you'll be forced to make a cover yourself, and they're not difficult to make!
Since I've always had 40's - 50's ironing boards, both wood and metal, I too found that the "standard" covers would never fit for various reasons.
If you do decide to make a cover, I've found that a tight weaved, heavy canvas cloth works the best (be sure to wash it it first in warm-hot water and dry it on the warmest setting. If the canvas doesn't shrink before your sew it, you'll have a heck-of-a-time trying to attach it to your board after you wash it once!). If you do make a cover, be sure NOT to use any foam padding. If the iron is too hot, it can melt onto the ironing board, or worse, catch fire. A very heavy cotton or wool batting material works best for your pad. You can find all of this at any local fabric store. I usually hit the remnant racks first, then meander through the various rows.