Lush Growth is a good card(see below answer), it's a green-based card; if you're playing green, than THAT's the best one to use, because it has a VERY low mana cost. Blood moon'll change any NON-basic land into a mountain, which won't help if
a) Your opponent doesn't use/isn't playing any non-basic lands, or
b) You don't have at least one red mana free to play it with.
A lot of people don't use nonbasic lands right up front, so you might get screwed on that one, depending.
You can also throw in some "Add one mana of any color to your deck" artifacts; then, if you have a card that isn't of a color you'd ordinarily have(i.e., like Lush Growth, above;) say it's in your hand, and you play something like Chromatic Star, etc; then you can add one green mana to your pool, and use it to play your lone green card.
I HAVE done this before, and if you're trying to build your entire deck AROUND insurance for your Mountainwalk, it's a nifty idea. But I probably don't have to tell you that the odds don't favor it quite as much... but then again, if you have any other card that'd do it, the max in any deck is four ANYWAY, so you know, four artifacts and four transform land cards of any color aren't much harder to draw out than, say, four(or one!) mountain-changing land.
If you want a blue-based card that'll do it, try Phantasmal Terrain: that's my current favorite. It costs two blue mana to play, and it turns any target land into any other land of your choice. I'd have to know if you're playing JUST a red deck, or what color combo you're using; but I can turn you on to a REALLY nifty little tool, if you don't know it already: the Gatherer, on the official MtG site. It allows you to search for a specific card by text, type, format, color, cost, or just about ANY combination you can bust your brain trying to come up with. You can find it here:
http://gatherer.wizards.com/
I checked really quickly to see if the card I was thinking of was a red-based card, but it's not. That should help a LOT, though.
There are quite a few cards that allow you to search THROUGH your library to FIND any color land you choose, and then put it into play on YOUR side; these are really worth it, and sound like something you'd might want in any case.
Plus, there are several other cards that allow you to manipulate the color of your lands. BUT: YOU HAVE TO BE CAREFUL THAT THE CARD ISN'T MEANT TO MAKE THE *MANA* you'd have from one mountain, i.e., "Add @ to your mana pool" or "Card X provides red mana along with its own basic color".... and not one to TRANSFORM a land of any player's, which you need to be able to Mountainwalk. (They have to physically control one.)
Also, remember that all of these changes *are* enchantments; so if someone comes out with a "Destroy target enchantment" or target permanent, or what have you, you'll lose your Mountainwalk as soon as they play it.
Hope that helps!