Ask the DM about Magic Items that Grant Feats
A character may, if the DM permits, be able to acquire the least invocation call of the beast (Complete Mage 123) using the Dungeons and Dragons, Third Edition (which remains legal in Dungeons and Dragons 3.5) Arms and Equipment Guide rules for Magic Items that Grant Feats, which says, in part, that
A general guideline for [determining the market price of] other kinds of [items that grant] feats is that they cost 10,000 gp, plus another 5,000 gp to 10,000 gp per prerequisite.
Thus a magic item granting the feat Extra Invocation (Complete Arcane 79) would, by this estimate, cost between 15,000 gp and 20,000 go because of the feat's prerequisite. The item's final cost, the item's prerequisites, and—most importantly for our purposes—whether the wearer of such an item must still meet the feat's prerequisite are up to the DM.
Gaining the effects of the invocation call of the beast
I only found one item that granted an at-will effect like the spell speak with animals [div] (PH 221), and that's the circlet of the wilderness (Unapproachable East 56) (50,000 gp; 0 lbs.), which grants the ranger that wears it a host of spell-like abilities, making reverse-engineering the author's evaluation of that specific effect practically impossible.
I hate Table 7–33: Estimating Magic Item Gold Piece Values (Dungeon Master's Guide 285), but without further guidance, it gives at least a starting point. Using the table to compute the price of a continuous 1st-level speak with animals effect yields a market price for such an item of 4,000 gp.1
Computing the price of a magic item granting the extraordinary ability wild empathy (PH 35) requires some Dumpster-diving. The mantle of rage ("New Magic Items" 2) (6,000 gp; 1 lb.) grants any creature that wears it the extraordinary ability rage 1/day as if the creature were a level 1 barbarian. I think for most campaigns that's a reasonable comparison and a reasonable price.
Were I DMing, I'd probably allow a player whose character possesses the feat Craft Wondrous Item to create a custom magic item (say, helm of the beast?) that grants a continuous speak with animals effect and the ability to use wild empathy as if a druid for a market price of 10,000 gp. In a campaign wherein such abilities are particularly useful (e.g. animals everywhere have long memories and incredible observational skills just no language, hiveminded penguins as primary opposition, The Herculoids, Jurassic Realms), I might as much as double the item's price, but I'd still keep in mind the cost of that circlet of the woodlands.
I'd probably make the prerequisites the extraordinary ability wild empathy as well as the spell speak with animals so the character'd have to work alongside a smelly druid for a week or three. Because that's funny.
1 2,000 gp base price ×1 for caster level 1st ×1 for spell level 1st ×2 for a spell duration normally of 1 min. per level.